engine rvs dropping

hi, i have a 1998 subaru impreza turbo with 56.000 on the clock.i have an annoying problem with the tickover,when i,ve been out on a run and come to a junction or traffic lights the engine rev s drop down to approx. 150/200 rpm then stabilise to normal after 5/10 seconds,occassionally the car stalls.had car on diagnostic but no faults showed up,any ideas would be appreciated,thanks