I suspect your problem may be the fuel. What is the specification of

your fuel? Lycoming Service Instruction SI1070S does not approve

any 92 grade aviation fuel. They approve automotive fuels with an AKI

(average of MM and RM octane ratings) at least 93 for the 0-540-B.

Check the SI for more advice.

If that's too low AKI - there will be detonation, vibration, high CHT\EGT ets.
And it will for all time. And not for 30 sec, no?

We worked on that fuel in summer, and have no troubles then...