My old Crapsman router would let the bit come loose every so often as it heated up. I tried cinching it tighter to no avail. I mic'ed the bit and it was dead on, but there were no marks on the shank, as if it had never been used.
I thought my collet was titsup, but other bits would stay in with no problem, only the ones that had high use in a cut were a problem. Hmmmm, sounded like a buildup of some sort.
I took the collet apart and let it soak overnight in a small jar of "BrakeKleen" from the local Napa Automotive store. The next morning I used a small brush and a rag to clean the parts up and Waa-Laa... Everything was happy in router land.
My theory, and I can't prove it, but after years of routing plywood or glue joints material, some of the glue had melted or collected as dust in the collet. This let the collet not tighten as it should and as it heated up, the glue would melt and let the bit slip.
Good Luck, I don't have as much experience as these other guys, but this is what worked for me.