Hate to be a downer but if you saw flashing then 100% it isn't dirty contacts and it needs attention. This is more likely arcing in one of the power tubes and quite likely caused as you surmised by switching off and then on too quickly. If the valves survived then they may be compromised and need proper checking (a simple valve tester may not show up the effect, you'll need a curve tracer). Or you could buy new and wait till the opportunity presents to find which have survived. I don't know the amp or the protection it has but better safe than sorry.

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