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Maybe the valves are to tight in the guides causing them to bind up when it gets hot.
That is good idea, I will check this!
I now try to remember, when pop off the shims, and maybe the pop off happend, when the engine is not enough warm.
I always test, when water and oil warm, but maybe the cylinder head, and valve guides not full warm, and when I start to go with full boost, the exhaust valves turn to be thicker, and seized a little bit.
Many times, when I drive to the competition on road, and there I start to practice slower, the car work fine during all competition, when I test it near to service, shims pop off after few km.