Almost one-third of Metro's subway cars were in the stop yesterday as motor breakdowns and other difficulties confronted the rail system with its most critical operating problems in almost three years.

The problems, steadily increasing for the last few weeks, forced Metro riders to wait several minutes longer between trains, to miss bus connections at transfer stations and sometimes to be left at the platform because shorter, less frequent trains were jammed with standees.

"It's so crowded on the train that a lot of people are really irritable," said Gwen Ness, a 19-year-old federal employe who was waiting yesterday evening at the Metro Center station for a New Carrollton line train. "I've had to run to work to keep from being late."

No immediate relief is in sight, according to Metro General Manager Richard S. Page. "This is probably the most critical problem facing Metro," Page said yesterday.

On Monday, Metro had only 202 of its 300 cars available for service on "the worst day I can remember," said Anthony J. Stefanac, Metro's rail superintendent.

Yesterday, 208 rail cars were available, which means Metro was forced to operate five fewer trains than it is supposed to have.

"They're breaking down almost as fast as we can replace them," said Erich Vogel, Metro's director of subway maintenance.

The subway car problem, according to many officials interviewed over the last few days, is a natural result of a Neanderthal-age parts procurement and maintence program for Metro's space-age cars. Transit systems elsewhere, for example, have been keeping maintenance records on computers for years. Metro, however keeps them on 5-by-8 hand-posted cards and has no quick way to determine breakdown patterns or to reconstruct the history of certain problems.

The maintenance problem is exacerbated by the fact that the manufacturer of Metro's 300 cars -- Rohr Industries of Chula Vista, Calif. -- no longer makes rail cars. Although more than half of Metro's 300 cars still are under warranty, "Rohr's responsiveness is really becoming weak," said Fady Bassily, Metro's rail car expert.

"Rohr has removed any (Washington) personnel and is operating the warranty out of the West Coast, which makes it academic."

Although motor problems are the current top priority, other issues also are interfering with subway service. The braking system is temperamental and always has been; if its electronic sensors don't like what they sense, they stop the train and it can take many minutes to get it going again.

Two weeks ago a full rush-hour train to New Carrollton was stopped for 30 minutes on the elevated section over the RFK Stadium parking lot. The train never did go. A "rescue train" was brought in and it took 40 more minutes to walk all the passengers through one train into the other. Both brake and motor problems were involved.

Air-conditioners, a critical component even in wintertime, have been beset with motor problems of their own.

Destination signs -- critical to passengers on the Blue-Orange line who want to know where their train is going -- are unpredictable. It is a cause for celebration if all the signs on one train agree.

"Our first priority has not been destination signs," said Vogel, the maintenance director. "Right now it's motors."

Finally, Metro has had a difficult time keeping qualified mechanics. At one point last summer, Vogel had 36 vacancies in the rail maintenance shop.

A computer has been ordered to keep track of cars and parts. An intern program has been started so Metro can "grow" its own mechanics. Other steps have been taken to improve supply procedures. But Metro still had a four- to six-week backlog yesterday in the handling of requisitions for parts. That backlog was first discovered in August.

Rohr, the rail car manufacturer, is pursuing a $50 million claim against Metro, charging that Metro forced it to absorb huge losses while the system's 300 cars where being constructed on a fixed-price bid of $91.8 million -- about $300,000 a car.

Furthermore, Metro's Bassily said, "you have to prove your case under warranty. Most of the time, we are not "in a position to prove it, because of the lack of solid maintenance data.

Rohr had no comment yesterday. "We're out of the business and a claim is pending," a spokesman said.

Metro has been forced to obtain parts from Rohr's subcontractors. For motors, the subcontractor is Westinghouse.

Westinghouse's subway motors -- one per axle, four per car -- were the reason that 76 of 88 out-of-service cars were in the shop on Monday.

There are two problems with the Westinghouse motors, both Metro and Westinghouse officials agreed yesterday.

Bearings are wearing out for reasons not entirely understood after about 50,000 miles. Metro expected about 200,000 to 250,000 miles per bearing. Replacement bearings have been slow in coming, despite the fact that Metro ordered them in plenty of time.

"Commutators," key parts that rotate constantly, are losing their roundness at an unusual rate and have to be re-milled. The problem causes the motor to short.

"The fact is," Page said yesterday, "We do not have the patient completely diagnosed."

"There is no miracle cure," said a Westinghouse official intimately familiar with the Metro problems.

Page said that Westinghouse "is the one corporation of the six or seven we deal with that is very cooperative."

Metro's cars have been plagued with problems from the start. When Metro opened the Blue Line in July 1977 and crossed the Potomac for the first time, passengers stormed aboard, the doors stuck open and the trains refused to budge. That problem has been fixed.

Nathanal Wood, interviewed yesterday as he waited at Metro Center for a train to New Carrollton, said, "The trains are slow. Three or four Stadium-Armory trains come by before you can get one to New Carrollton. The last couple of days I have waited for 10 to 15 minutes up to half an hour. It's usually about a two- or three-minute wait."