Coming off an emotional revenge victory over a despised rival, the New England Patriots should be ripe for a letdown.

Don’t count on either of those scenarios when the teams meet on Sunday in Foxborough, Massachusetts.

Dallas has been one of the most up-and-down teams in the league. Things don’t fluctuate from week to week with America’s Team, they do so from play to play.

That’s no way to perform against the relentless Patriots, who haven’t been dominant in a 4-1 start to the season, but are far steadier than Tony Romo and company.

“You never want that to happen,” Romo said of Dallas’ split personalities. “We were also fortunate to have all the things go right against Washington and against San Francisco. For every game that’s the other way, there’s another on the other side. Everyone is good in this league. You have to minimize turnovers in key situations. We did that for two of the games and two of the games we didn’t. That’s really what it comes down to.”

What this could come down to is whether the Patriots hit their stride in gaining their measure of vengeance against the New York Jets last weekend. It wasn’t a rout, but it sure felt good to New England after the way New York won their playoff matchup last January.

One thing is certain for this game: Dallas must find a way to slow down Wes Welker, who leads the league with 45 receptions and with 740 yards receiving, the most in NFL history through a team’s first five games.

“I don’t think you can really look at them and not think this is a little bit crazy,” Welker said of his numbers and of quarterback Tom Brady’s 1,874 yards, and 14 touchdowns through five weeks. “But at the same time I try not to really think about it. Just try to get open and catch the ball and play for my team.”

New England has won the last three meetings after Dallas took the first seven in this infrequent rivalry. The Patriots also have won 19 consecutive home games and 22 of the last 23 against the NFC. So even if the good Cowboys show up, it might not be enough.

Elsewhere on Sunday, the two unbeaten teams have entirely different opponents. The Detroit Lions host the San Francisco 49ers, who sit atop the NFC West at 4-1, while the Green Bay Packers are home for division tailender the St. Louis Rams.

If both the Lions and Packers get to 6-0, it would be the second time since the NFL split into divisions in 1933 that two teams in the same division started with six wins. In 1934, Detroit and Chicago each started 10-0. The Bears went on to lose the final to New York, while the Lions finished second in the division.

In recent years, Detroit vs. San Francisco would have been a dog of a game between mediocre (or worse) clubs. Now, it’s one of the highlights of the weekend and a sellout.

The Niners won both of their road games and are a late blown lead against Dallas from also being unbeaten. They are buying into new coach Jim Harbaugh’s approach, and it showed in particular with a huge comeback win at Philadelphia, then a rout of Tampa Bay at Candlestick Park.

“I think everybody recognized that football was going to be in his future in some way when he wasn’t playing anymore,” Lions coach Jim Schwartz said of Harbaugh. “(He’s a) very good leader. He played quarterback like a coach would.”

A very good sign for Detroit was how it didn’t play lights-out football on Monday against Chicago and still won. How San Francisco’s ball-hawking pass defense deals with Lions receiver Calvin Johnson could be the key.

The unbeaten Packers facing the winless Rams looks like the mismatch of the season. The Rams, coming off a bye, have been abysmal on defense and even worse on offense. The defending champion Packers, even when not clicking in all facets, win games.

Might Green Bay be headed into a trap?

“My message ... to the team was there’s a difference between being real confident and being overconfident,” coach Mike McCarthy said. “Mature football teams play with real confidence and immature football teams play with overconfidence after coming off a big win as we had in Atlanta on Sunday night. So it’s important for us to spend more time on the Rams than we normally would.”

It’s also important for the Rams to find a way to slow Aaron Rodgers and his deep collection of receivers. Rodgers has thrown to 12 teammates already this season.

Also, it’s Buffalo at the New York Giants, Houston at Baltimore, New Orleans at Tampa Bay, Philadelphia at Washington, Jacksonville at Pittsburgh, Carolina at Atlanta, Indianapolis at Cincinnati, Cleveland at Oakland and Minnesota at Chicago.

On Monday, Miami is at the New York Jets.

Off this week are Arizona, Denver, Kansas City, San Diego, Seattle and Tennessee.