I have a 1991 940Turbo wagon that keeps stalling at random moments. It was fine until I had an oil leak repaired and the mechanic power-washed the engine to get the old oil off. Since then, it has stalled in traffic and while driving on the interstate at 65 mph. I have had it back to the mechanic (who specializes in Volvos)and had fuel system relays replaced twice. Any ideas?