1994 VW Golf mileage: 150,000. I am experiencing a problem with the warm up phase. The engine will increase to 2500-3000 revs as it warms up. It will then stay at this level for anything up to thirty minutes and then drop back to normal (suddenly). For no apparent recent the engine will now and again suddenly start racing again only to drop back to normal just as suddenly. Any ideas? I wonder if the culprit is the Stage II vacuum delay. If you have no probable solution, I would be happy with a list of the units or components that could be responsible.