Volvo 960 missing

I have a 95 - 6 cyl. In the past year, if I hit a very sharp bump, the engine will run rough - like it only has spark on 2-4-6. If I come to a stop, the engine will reset and run OK. Recently it has started to do this making right hand turns. It is always much worse when the engine is cold and it is below 35 degrees outside. Any suggestions?