Beyond the afore mentioned checks, yes vacuum leaks can behave this way too, also check that all plugs are firing consistantly, an inductive timing light w/ RPM reading works good for this. Clamp onto each plug wire, and check that the RPM/Flash rate is consistant with the Tach. Try to listen for a solenoid switching when the rev changes, there are 2 of these guys under the intake that are almost identical and easily mixed up when hooking the vacuum lines back up. I would have to check my shop manual for the correct hookup.
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