Frustrating  1% of the time at cruise 5000-5200 rpm, getting worse. 912 ULS on Rans S7 amphib. About 12 hours since rubber replacement including carb overhaul , sprag clutch replacement, and gearbox inspection. Electric and mechanical fuel pumps also recently replaced. New-to-me aircraft ran perfect on 3-hr flight home from mechanic that that did work above.  Next flights had 1-2 seconds of vibration and 100-200 rpm drop that cleared with no input. Then next flight had 300-400 rpm drop and vibration that lasted longer and cleared as soon as electric fuel pump turned on. RPM’s started dropping as soon as fuel pump turned off.  Then next flight over the field ran well without electric fuel pump until 500 rpm drop and more severe vibration that took 3 seconds (felt longer!) of fuel pump on to smooth out. Thanks for suggestions.

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