What George says makes sense, of course. Fluctuating vacuum gauge like that can be a burnt valve or stuck, also. After doing George's suggested checks, a leak down test would further isolate the issue, if you still haven't definitively found it. Time since last valve adjust isn't as important as miles, assuming there isn't an issue with a stuck valve due to varnish from lousy or old gas. Normal interval by the old Chevy documentation was 5,000 miles between engine tune-ups, which included valve adjustment.
You may already know all this... but here goes. Compression should be about 130 # with all readings within +/-5% of one another. One or more cylinders that are "out of family" will help isolate the problem area. If you have one or more low compression cylinders, a squirt of motor oil through the spark plug hole and retest will tell you a lot. If the compression comes up after adding the oil, then it suggests stuck or worn rings. If the cylinder under test is still low and doesn't respond to the oil, that suggests a valve seating issue (burnt or stuck).
For context, I should add, I'm thnking that you may have a stuck valve, since the recent fill-up was mentioned. If that loosened crud in the tank, it is possible that the crap released contributed to a stuck valve...
Hope this helps.
Last edited by drdoug; 04/07/2019 8:00 PM.
