i just bought a 95 f3. after i drive it for 15 minutes or so espwcially hei´m in 4th or 5th gear i go to give it gas and it starts to stall out. i hve to shut it off, move gthe choke lever around and itsarts no problem. goes bc to normal. it will do it again in 10 minutes. is it the carbs that are dirty o something else?