88 gmc loping

Hello all, I've got an 88 gmc full size 350, 5.7, pickup. It recently started to lope. By that I mean, at idle it seems to want to stop running, it does not. When I give it gas and rpms are up, it runs fine. I have done vacuum tests on the egr solenoid and valve. They both seem to pass. Timing was good. What am I missing? Thanks for any help, Mp

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