88 S4 NA has high voltage spikes or surges that can last up to 10 seconds. The surges will cause electrical feedback noise in the speakers. Starting or idling isn't a problem. Cruising at 2k RPM is also not a problem. This situation only happens under acceleration, but the high voltage effects can last after the revs have dropped. So far I haven't fried anything...

I've had the alternator checked out both on and off the car. It's fine. But when it was checked on the car, I was told that power is not getting back to the battery.

I've cleaned up the ground wires on the battery, starter solenoid, and driver's side shock tower. This site was used as a guide: http://www.aaroncake.net/rx-7/grounding.htm

The search results I have found on this forum tend to lean towards grounding issues or bad alternators.

Can anyone be of assistance?