After the bonus of a 50-foot birdie putt on the difficult fifth hole, Northern Ireland's US Open champion carved his next shot into the trees.
Then, after bogeying that hole, he hooked his drive down the seventh into the lake and dropped another shot.
Worse was to come. Going for the green in two on the long ninth he hooked again into a small bush and, seeing no better option than to have a thrash at the ball, moved it only a few feet.
The next shot found rough and although he got up and down it meant a bogey six and an outward 38 that put him three behind the leader along with Nick Watney and Paul Goydos.
Toms, meanwhile, had three birdies in his first six holes, the last of them a pitch that finished just two inches from the cup.
The wonder at that point was how on earth the 2001 US PGA winner had gone more than five years without a victory and had fallen to 75th in the world.
But he failed to get up and down from sand on the short eighth and missed a birdie chance at the ninth.
Watney posted a front nine 35 that promised so much more when he birdied the first three holes to reclaim a share of the lead.
Bogeys at the eighth and ninth hit his hopes of adding the title often dubbed the sport's unofficial fifth major to his Cadillac world championship in Miami in March.
Luke Donald, needing to win to take the world No 1 spot for the first time, managed only level par for the outward half and was four back.
He hit approaches to within two feet on the fourth and seventh, but they followed bogeys at the long second and sixth, where he raced a 22-foot birdie attempt nearly nine feet past.
McDowell and Donald, wearing navy blue in memory of the late Seve Ballesteros, were both trying to become the third European winner of golf's richest event - which brings a top prize of more than $1million (618,000) to the champion - in four years after Sergio Garcia in 2008 and Henrik Stenson 12 months later.
Earlier, McDowell had completed his third round following a delay to the tournament because of a thunderstorm on Saturday. He moved one ahead going into the final round but it could have been so much better.
A month after his Ryder Cup partner Rory McIlroy went from four ahead to ten behind with a closing 80 at The Masters, McDowell was three clear after birdies at the 16th and 17th.
But the Northern Irishman had a stroke of misfortune at the last when his approach to the last kicked off the bank of a bunker and rolled all across the green into the lake.
"I think I was a bit unlucky at the last," said McDowell before the final round."When it landed I thought 'perfect', but then it started going down the slope and I thought 'trouble'.
"I guess it's a tough break, but I played great and I just want to have a chance coming down the stretch. That would be a lot of fun."