Houston, we have a problem. At least, that's how Pádraig Harrington's scorecard looked as he dropped six shots in his last five holes in the third round of the Shell Houston Open.

A triple-bogey-par-double-par finish (7-5-3-7-3) added up to a homeward nine of 40 blows for the Dubliner, who saw all his good work undone as he shot a two over 74 to crash from the top 20 to tied 59th on three under.

He's 11 strokes behind Jordan Spieth who shot 67 to lead by one from Scott Piercy (66), Johnson Wagner (66) and the unheralded Austin Cook (70) on 14 under par at the Golf Club of Houston.

Birdies at the 12th, 13th, first and fourth left Harrington sitting pretty and pushing for the Top 10 in Humble.

But he hit a poor drive and made seven at the par-five fifth, bogeyed the sixth after putting his approach into a lake and the duffed a chip and took three more to get down after overshooting the par-five eighth (his 17th) in three.

Phil Mickelson also had a disappointing, finishing with three bogeys for a 75 that dropped him from second to tied 18th on eight under. 

The highlight of the day was an ace by Patrick Reed at the 16th followed by his race to the green with his caddie.