FOXBORO — Momentum is a fleeting thing.

Over the last few weeks, the Patriots have had plenty of first-half momentum, rolling to early leads over Miami, Indianapolis and the New York Jets. Against the Dolphins, New England held a 16-10 halftime lead. Against the Colts, they were up 24-7 in the second quarter. And against the Jets, they held a 24-7 advantage at the half.

But those leads shrunk in the second half. The Patriots won two of those three contests, but the same second-half struggles New England suffered in those three games have dogged the Patriots’ offense all season. In fact, all three of the Patriots losses — in Week 2 to the Jets, in Week 5 against Denver and Week 10 against Indianapolis — can be traced back to what quarterback Tom Brady on Wednesday called “a few bad halves of football” by the New England offense.

“There have just been little lapses over the course of games — like last week before the half and into the third quarter — that we have to eliminate,” Brady said.

All three losses have been close — three defeats by an average of 3.7 points. Which means that sometimes, the little things have meant a lot.

“I think it’s a collective thing,” said New England director of player personnel Nick Caserio. “Whether it’s a route here, a dropped ball here, maybe a missed block. So I think you’re trying to eliminate collectively as a group some of those little things that may cause you to stall a little bit.”

New England is still the third-highest scoring team in the league, and the No. 1 in the AFC with 290 points through 10 games. Only New Orleans (369 points) and Minnesota (306) have put more points on the board this year.

But the Patriots have struggled offensive in the third and fourth quarters this season, putting up just 94 second-half points, including just 39 in the third quarter. Those numbers stand in marked contrast to the 196 first-half points (125 of them in the second quarter) they’ve amassed through the first 10 games of this season.

“Football is a four-quarter game and you can’t relax for any portion. I’m not saying we’ve relaxed, I’m just saying our execution over the course of those four quarters wasn’t what it needed to be,” Brady said.

When the Patriots have been at their best, they’ve put together four impressive quarters of football — the games against Tennessee and Tampa Bay come to mind as the two most obvious cases where New England has kept their offensive momentum through all four quarters.

When they’ve laid off the gas pedal — and the opponent has taken advantage — it’s cost them. Brady said Wednesday the Patriots need to start flashing the killer instinct they displayed on many occasions this season, including the second-half of the 59-0 win over Tennessee.

“The momentum changes can happen very quickly,” Brady said. “Once you lose the momentum, it’s like playing a different team the next time you go back out the next series. It’s important when you have a team that’s reeling; you have to keep piling it on so that they don’t have an opportunity to come back, like we did against Tennessee. We just kept scoring and scoring.

“We’ve just had some games this year where we haven’t done that and the other team has gotten back into the game, and we’re the ones trying to claw back at the end. Believe me, it’s something we’re all focusing on, something that has been brought to our attention on a daily basis by our coach.”

Much of their trouble takes place on the road. In three road games this season — setting aside the “road” game against Tampa Bay in London — the Patriots have scored just 10 second-half points. After taking leads in all three of those games, they were blanked in the second half against the Jets and Broncos, and managed just 10 second-half points in the loss to the Colts.

“Playing 60 minutes of football, I think that’s what every team is looking to do in the league — to play consistently over the course of every play, every series, every punt, every kickoff, every offensive series, four-minute, two-minute, the situations,” Brady said. “You have to be sharp. You have to be on it.”