.Did you have the manifold checked foe a straight surface?It CAN still leak with a new gasket.
.How does it run when it is cold? does it do the same thing?
.I would check and set the point dwell and the timing,then put a vacuum guage on it while running and warmed up.should be around 17 to 20 in.Hg on the guage.
.Make sure you set the dwell first and the timing second.

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