I bought a 1988 230 really cheap with a view to doing a rolling restoration. One thing I cannot put my finger on is a strange sound coming from the engine. It sounds like a buffeting almost like a non firing cylinder. The car drives OK up to about 110kph and then gets sluggish. Can't use a tester a I broke a pin (no 3) on the tester - waiting for new replacement. Anyone experienced something like this?