timing dances around after warmup

89 pickup....just had a valve job and head gasket replaced. Timing is perfect until the enging warms up, then with the timimg light atatched you can see it dancing back and forth.
any help would be apreciated dave
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D. Kocher wrote:

When was the last time the distributor cap was replaced? I assume this truck has electronic pickup and no points?
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the cap and rotor were just replaced....yes, electronic

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Only other item that comes to mind is the cam angle sensor. I don't know if thats on the engine block somewhere or very commonly in the distributor.
D. Kocher wrote:

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thanks....i will look into it

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