My 1990 750iL (about 70K miles, nice condition car) started running rough about 6 months ago. It would start and idle fine, but accelerating from about 2000 rpm it would miss and shake a bit. I replaced the plugs. It ran better for a few weeks and then the miss/shake came back. So I replaced the plug wires (and reset computer codes per Shogun) and again it ran OK (not like new) for another few weeks and the shake and loss of power comes back. So I replaced the O2 sensors and reset the codes again and it has been stellar for 6 weeks, great accaleration - fixed, no. Yesterday coming back from a 70 mile trip from home it started up again to the point were I barely got home. No power and extreme vibration during acceleration only. Starting, idling, low demand on engine its fine. How can a car be fixed and go back to its old ways on its own? No engine or EML light on. Any ideas? Thanks.