this can happen if the iscv has been disconnected (normally to set base idle) then reconnected & not left to idle for 10 mins before reving/driving (for ecu to reset its new idle values) have you cleaned out the throttle body inc the passage of the base idle screw? also whats the base idle set at? (700-750rpm). to check it yourself for air leaks (if in doubt!) spray carb cleaner around the inlet area. (ive always found they drive crap after having iscv disconnected, but get better after a couple of weeks, but this may just be on mine!)