GUINNESS PREMIERSHIP leaders Gloucester suffered a setback as second-half replacement George Robson snatched victory for Harlequins with a 75th-minute try.
Robson applied the finish and fellow substitute Chris Malone added the conversion to round off a late surge by Harlequins who recorded their third successive victory.
Poor goal-kicking by fly-half Willie Walker and a malfunctioning line-out proved costly for Gloucester.
With Scotland’s ace marksman Chris Paterson restricted to the bench, Walker managed only two successful kicks from seven attempts.
Tries from England full-back Iain Balshaw and three-quarters Jack Adams, Anthony Allen and James Simpson-Daniel should have been enough to see the visitors home in an error-strewn match.
But Harlequins kept in touch with tries from prop-forward Ceri Jones and full-back Mike Brown, with a 13-point contribution from fly-half Adrian Jarvis, who converted twice and also kicked three penalties.
England winger Lesley Vainikolo was the only Gloucester three-quarter not to get on the scoresheet, but his powerful running caused Harlequins plenty of problems in the first half before he faded as the game went on.
There were five tries before the interval as the defensive shortcomings of both sides were exposed.
Jarvis kicked Harlequins ahead with a 14th-minute penalty but the fly-half paid the price when his kick was charged down in his opponents’ half.
Gloucester centre Adams took full advantage, snatching up the loose ball and out-stripping the home side on a 70-metre run for the line.
The lead lasted little more than a minute. Quins winger Ugo Monye took a brilliant catch from the kick-off and charged to within a metre of the try-line but the home side were not to be denied as they forced Jones over for the try.
Three minutes later Gloucester grabbed the lead again.
Vainikolo began the move with a trademark barnstorming run from inside his own half and offloaded as he was tackled for Balshaw to race over.
Walker, Gloucester’s third fly-half in as many games, converted and then kicked a simple penalty to extend the lead to 15-10 before combining with Simpson-Daniel to send Allen racing over in the left-hand corner.
But, a minute before the interval, Gloucester were caught napping as Brown took full advantage of a huge defensive gap to run unchallenged through the middle for a try between the posts.
Jarvis landed the second conversion of the half to cut the deficit to three points at the interval.
The fly-half’s second penalty levelled the scores at 20-20 in the 54th minute.
Simpson-Daniel earned Gloucester a bonus point with their fourth try when he finished off a slick three-quarter move in the 58th minute.
But Jarvis took Harlequins to within two points with his 69th-minute penalty before the final surge which produced victory for Harlequins in front of a sell-out crowd of 12,638.
Gloucester replaced hooker Olivier Azam, whose lineout throwing had been erratic, with Andy Titterrell after 51 minutes, but their pack had lost their momentum as the home side capitalised on Walker’s misses to take control of the match in the final quarter.