2003 Vauxhall Signum engine management light coming on then safe mode

had the car 3 months under warranty occasionally comes up with engine management warning light (spanner on a car). fault codes show P0638 which shows up
electronic throttle positioning sensor circuit high input
and throttle control circuit negative deviation.
The garage cannot find anything because its intermittent and of course not on when I put into the garage . Symptoms are going along constant speed normally after engine warmed up there is a light jolt like coming out of gear ( its and automatic) then fault light comes up sometimes goes into safe mode sometimes carries on and light goes off . Happens in "flurries" 10 light shows then back to normal for 200 miles or so . Dangerous on motorway!!