METRO'S SUDDEN fit of paralysis yesterday morning was particularly unfortunate. A computer at Rosslyn suffered a nervous breakdown - brief and curable, as it turned out, but for the Virginia commuters a painful one. It came just at the beginning of the morning rush, and took a full hour to fix. While it lasted, it blocked rail traffic completely in both directions. Metro's management scrambled to line up buses to carry thousands of stranded customers over the bridges.But once again, a lot of people were late to work.

No reasonable rider would have expected a system as complicated as this new subway line to go into operation completely smoothly. Metro is entitled to a certain grace period in which to iron the wrinkles out of its exceedingly elaborate control and safety systems. After all, the Rosslyn computer erred on the side of caution as it had been taught to do, and that is infinitely preferable to the other possibility. But there have been a great many of these malfunctions on the Blue Line since it came into service six weeks ago.

Metro does not have unlimited time in which to begin delivering the quick and reliable service for which those billions of dollars are being spent. Word is beginning to get around that, if you're in a hurry, you'd better not depend on the subway. Metro's chief assets have always been the hopes of taxpayers and commuters that the rails will provide a faster, more reliable ride than the bus. Our sense of it is that Metro is drawing down those crucial assets at a dangerously rapid rate. No irreparable harm has been done yet, and late summer is not the worst time of year for a shakedown of the new equipment. But if the present pattern of unpleasant surprises should continue day after day into the autumn, when the tempo of life picks up again, the accumulating disappointments and inconveniences will cast a heavy shadow over Metro's future.

Very few people can afford to be late for work over and over. As more people use the subway, the cost of each delay will rise, in terms of people's time and tempers. In early September more of the VIrginia bus routes are to be curtailed, requiring their riders to use the subway. Later in the autumn, the Red Line is scheduled to be opened to Silver Spring.It would temporarily increase costs, but perhaps Metro ought to consider delaying these actions until the present system has been brought to a higher level of efficiency.