I have a 1983 E30 M10 engine with the Bosch K-Lamda continuous injection system, all original. The car idles nicely with the occasional hic-up and accelerates and drives very nicely through first second and third gear, when you put the car into fourth or fifth the problems start. When you put the car under load in 4th of 5th the car accelerates well and performes normally but as soon as you reduce the load on the engine to coast the enging runs very roughly, if you then change back to third the car runs smoothly and normally again. There is no rough running in 1st 2nd or 3rd under any driving conditions. I have checked the ingnition system fully and it is fine, any ideas what to check next?