440 overheats once and then it's happy all day

You could always replace the thermostat with a washered opening. I know that I've seen charts on what size an opening would be appropriate if you decide to try that route.

I think I'd try running without a thermostat first to see if the behavior stays the same. If it does stay the same then you might be able to conclude that the thermostat 'not opening' is not the issue.