SAN FRANCISCO – It was the sort of no-nonsense game plan the 49ers had abandoned in recent weeks.

On offense, they jabbed away at the Eagles with running back Frank Gore and made sure tight end Vernon Davis got his shots in, too.

On defense, they scrapped the dubious "Big Sub" package that had failed them in recent losses to New Orleans and New England and returned to their base system. And through three quarters, the plan worked.

Then the fourth quarter reared its ugly head.

Quarterback J.T. O'Sullivan turned the ball over three times in the final quarter, and a 26-17 49ers lead melted into a 40-26 loss, the 49ers' third consecutive defeat. With Arizona beating Dallas, the 49ers are two games behind the Cardinals in the NFC West.

Afterward, O'Sullivan, who has thrown seven interceptions in the past three games, was asked whether his confidence has been shaken.

"It's good," he said. "I feel I threw the ball accurately. I want to win. That's the part that's the most frustrating. If you can't get past that, then everything else stops with the loss."

Coach Mike Nolan dismissed any notion the 49ers would turn to backup Shaun Hill.

"No, he's still the guy," Nolan said of O'Sullivan.

The 49ers have been defeated by wide margins the past three times they have faced the Eagles. What made this one particularly bitter was they were in control late in the game.

The Eagles took an early lead on touchdowns by running back Correll Buckhalter and wide receiver Hank Baskett. But with one second left in the first half, the momentum swung dramatically toward the 49ers.

The Eagles were attempting a 54-yard field goal that would have put them up 20-9. The attempt was blocked by defensive lineman Ray McDonald and scooped up by cornerback Donald Strickland, who ran untouched for a 41-yard touchdown.

"It just took a great bounce and landed in my hands, and I took it the distance," Strickland said. "It definitely carried into halftime. It was a momentum change – a 10-point swing."

Down by only one point to start the third quarter, the 49ers continued the ground attack with which they began the game. Gore (101 yards on 19 carries) had a 25-yard scamper on the opening drive of the second half in which he broke three tackles. On the next play, he broke two more for a six-yard touchdown. A Joe Nedney field goal on the following drive put the 49ers up 26-17 with 3:05 left in the third.

But the 2008 49ers have had trouble stopping opponents late in games, and that trend continued Sunday. The Eagles scored every time they had the ball in the final quarter – three field goals and an offensive touchdown, and also an interception returned for a touchdown – and the 49ers' nine-point lead turned into a two-touchdown deficit.

Afterward, Nolan noted that a year ago, it was the 49ers' offense that abandoned the defense. This year, the roles are reversed.

"Like I said, I think we changed the problem," Nolan said. "We've got to get our defense on track."

Players said the culprit was a familiar one: They allowed the Eagles to covert too many third down attempts – six of 12 – on the afternoon.

"We need to get them in third and longer (situations) so they don't have the whole playbook to mess with, so that one's on us," defensive lineman Justin Smith said. "For whatever reason, we couldn't do it. Screen or draw or whatever they were running, that doesn't matter. We've got to make plays. And we didn't make enough plays."

If the defense was bad on third down, the offense was even worse in converting just two of 12 of its third-down attempts. O'Sullivan and the 49ers ventured into the red zone four times and had to settle for three field goals.

"I get frustrated not scoring touchdowns," O'Sullivan said, "especially when we get the ball down there. It's our job to score touchdowns. Not doing that, we are leaving points out there."

Read Matthew Barrows' 49ers blog at www.sacbee.com/ninersblog.