My E39 M5 which I have had for 20 years, has developed an intermittent fault with even power delivery. This seems to happen if, say, I stop for petrol after driving for about an hour. On restarting the journey, the ride can become a bit lumpy, as it you were twitching the accelerator pedal. Sometimes it clears. The issue is usually not present the following day. I've changed the filters and the air mass sensors which have not made any difference. One theory is that this is build up of a carbon deposit inside the air flows which would require an extensive rebuild? I'm wondering if it is a failing electrical/computer part which struggles when getting hot and cold? Anybody has a similar issue?