CHaingKaiShek,
I had something like this happen on my 85. A few things I would try starting with the easiest:
- Trace the stereo wires and make sure it's grounded
- Try a distortion filter that hooks up to your stereo
- Check the NEG. wire on your battery & make sure it's grounded
- Try to relocate the alternator ground to the block or frame
- You could try to step up to a stronger alternator
You could also call the GM service center and they might be able to help you trouble shoot it over the phone (can't beat FREE profesional advice).
Let me know if any of this helps.
