I picked up a 1989 California 420 SEL recently that had sat for a number of years. Known problems were 1 bad fuel pump and the aftermarket stereo works at idle but as soon as I get start driving it cuts out, slow to a stop and it starts working again (OVP relay seems fine. I drained the fuel tank, replaced the fuel pumps, fluids, gave it a tune up and the car ran great at first. Then it started to run bad on less than 1/2 tank of fuel. Then it started to run really rich. Bad enough that I had a small cloud of black smoke around me when driving. Then it started to run rough even with a full tank of fuel. I pulled the plugs and they were all covered in soot and the #4 plug was carboned up, Valve stem seals I figure. Replaced the plugs, changed the oil and the car ran good for about a week, then started running rough again. So I pulled the plugs expecting to see carbon buildup again but they were fine, except for being soot covered again. Oil is black with less than 300 miles on it. Now the car runs like crap all all the time. Any ideas as to:
1. Why the car ran like crap on less than 1/2 tank of fuel
2. Why did the car run fine for a month before starting to run rich
3. Why does the stereo cut out

Thanks for any help offered!!