Thanks Sixto for the reply.

What is, and where is this "centrifugal IP timing sprocket" thing. That sounds like something that could cause this problem. I was told by the PO that the IP had been replaced when the vacuum pump broke and distroyed the timing chain and IP. The head was also rebuilt and new IP, vacuum pump, timing chain and guides. This was about a year ago, but it was not used much after the repairs. When I got the car it had a lot of problems, most of which have been fixed.
Thanks again for any help in this matter.

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84 500 SEL (307,xxx miles)