Buy a $3 tube of heat sink grease before putting it back on. Just the heat transfer properties or the grease should show a definite improvement if that's the issue.
Almost success. The H3 has only restarted twice so far. I still have about 50 shows to go. I did add thermal paste to the heat sync and that seems to slow the frequency of reboots but didn’t cure that problem.
If you can find something small and heavy (dense) to sit on top of heatsink to help it make better contact that will help. Then put a fan right next to it. That should keep things under control and not have any reboots at all hopefully. Then can get recordings transferred.