valve problems

I have a 98 yz400. The left side intake valve has stripped the cotters on multiple occasions. I can't be sure if the valve is sticking and causing this or if the valve just becomes stuck as a result of being struck by the piston. each time this happens I replace the valve and inspect the cylinder head but can't find any visible defects. Bottom line......stripped cotters, stuck valve, valve bent from piston. Not sure in what order. Please help, this is driving me crazy.

What does the valve seal look like when this happens? I'm thinking the valve guide is backed out or too long. Are you running high lift cams?

everything is stock on the heads. the seals and guides look good and everything feels and works perfect when i reassemble with new valves. I thought it might be a oiling problem

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