What a difference a game makes. The Washington Capitals, with that explosive six-goal second period that chilled the blazing New York Islanders on Sunday night, must suddenly be considered solid contenders for a National Hockey League playoff berth.

While the Capitals' 7-1 victory was cutting the Islanders' winning string at seven, both Edmonton and Vancouver were losing home games. As a result, Washington is only four points away from 16th place, the final qualifying spot in postseason play.

"We almost have to look at the playoffs optimistically after that one, which is something new around here," said captain Ryan Walter, whose two goals and three assists led the Washington attack. "This is the time of year when everybody gets the blahs, but it's also the time of year when you have to come bouncing back from that January after Christmas letdown. This is a long season and you need something to give you a lift. I'd have to say that game gave us one."

Outplaying the Islanders' all-star line of Bryan Trottier, Mike Bossy and Clark Gillies, by five equal strength goals to one, gave Walter, Mike Gartner and Bengt Gustafsson an injection of confidence. At the same time, defensemen Rick Green, Paul MacKinnon, Alan Hangsleben and Pierre Bouchard were proving something, too.

In the absence of flu-ridden Robert Picard, and with Hershey unable to furnish a replacement, that quartet went all the way, one shift on and one off, in a superb effort.

Green was on the ice for six Washington goals and in his last seven games has been a plus 12. MacKinnon, one of five Capital scorers, thrilled his visiting parents by producing his first NHL goal and two assists. Hangsleben drew rare raves from Coach Gary Green with another spirited performance, as his full-season rating climbed to plus 11.

Bouchard, nearing his 32nd birthday and rusty after a year's layoff and two early-season injuries, was faced with his fifth back-to-back assignment of January and the first in which Washington went with only four defensemen. Previously, the second game in two nights had left him gasping for air. This time, after a masterful performance, he seemed hardly winded.

"I think it's coming back a little more," Bouchard said. "Before, the second night I'd be stepping on my tongue. Now I feel good and I'm able to take a check better. I'm starting to hit a little more, too.

"I knew it would take a while. I've only played about 20 games in two years. I had a fairly good start at training camp, but I reached a peak and couldn't get my strength back. Then one injury, and another, and I missed six weeks and had to play a little slower. I should get stronger and I hope I get better.

"I'm glad I decided to play again. The crowd here makes you want to play and win some games for them. There have been some awful times here and we want to show the people they haven't been patient for nothing."

Goalie Gary Inness rejoined the Capitals yesterday after two weeks in Hershey . . . Peter Scamurra returned to the Bears, who won two weekened games with forwards Claude Noel and Bob Girard teaming up as one of the two defensive pairs . . . Goalie Wayne Stephenson is 35 today . . . General Manager Max McNab is in Miami for GM's annual meeting . . . The Capitals will resume play Wednesday night in Chicago.