After five years of some success -- 50 victories and two trips to the National Football League playoffs following a 3-11 season -- the New England Patriots are struggling again.

"Two years ago," Coach Ron Erhardt said before this season began, "we missed the playoffs by a game. Last year, we missed by a play."

Now, however, a 2-5 Patriot team is going to RFK Stadium in Washington to play the Redskins Sunday.

The Patriots, who lost their first four games, are all but mathematically eliminated from playoff contention. They could win their next nine games and still not qualify. And is anybody really thinking that they can win nine straight?

"A lot of things can happen," Erhardt said before the Patriots upset Houston Sunday. "We have to win eight of the next 10 games. If we do, we have a shot. We have a better schedule than the teams in our division and we still get two shots at both Buffalo and Miami. We can still make it."

Despite the season being nearly half over, neither Erhardt nor any other Patriot official has chosen to comment publicly on the scope of the problem.

With chin up and upper lips stiff, they won't discuss the season in the past tense -- at least not publicly.

Each week, Erhardt has come up with a reason for losing. Against Baltimore, it was a drive-killing offside penalty. Forgotten is that the Colts rushed for 270 yards.

There was a fumbled punt here, a blown field goal there. There's been a roughing-the-passer, an interception, blown coverage.

The fact remains, the Patriots have been losing.

They've been losing because they don't have players good enough to win with regularity.

Erhardt, however, said, "We're a good team that's not playing well."

Good teams, however, don't lose four of their first five games.

New England's slide may have begun as many as three years ago when it overlooked the "advancing youth" on the defensive line.

For years, the line screamed for new blood. But while General Manager Bucko Kilroy drafted an occasional defensive lineman, none seemed to work out. Two of last year's models, Steve McMichael and Doug McDougald, were cut this year.

And there were others who came and went, leaving Defensive Coordinator Fritz Shurmur with what he inherited.

"We don't have any individual stars," Shurmur says, "but as a group we can get the job done."

The job hasn't been done.

Julius Adams, 34, a fixture since 1971, made the Pro Bowl last year, but hasn't played well this year. Nose tackle Ray Hamilton is 31, Tony McGee 34 and Richard Bishop 31. And, excluding young Mark Buben, they are it.

Teams have run on the Patriots for an average of more than 200 yards a game. Not only can't they stop the run, the Patriots don't have much of a pass rush.

The long holdout of fullback Sam Cunningham contributed to the slide, as did the retirement of tight end Russ Francis. Certainly, the trading of tackle Leon Gray to the Oilers didn't help.

It's been gradual, the Patriot slide that developed into a full-blown crash this year. They've lost good people for one reason or another, and they haven't filled the holes sufficiently.

The Patriots, barring a miracle, are finished for 1981. Further, the future doesn't appear overly promising. New England has players who cannot do it any more, and others who never could.