I like the hockey pucks idea! I'm going to use that if I have any inkling at all there's a heat issue.
Glad yours is working well.
Freeze issues could be anything, but heat, given the random / non-repeatable nature of those reported here seems like a probably suspect. If it was a lack of memory, lack of processing power, problem with a particular disk, etc... those things should all be very repeatable. Heat buildup is very unpredicable.
I suppose another potential probably suspect would be if the players have a sub-par power supply. Comparing them to PCs, flaky power supplies can cause no end of tough to reproduce instability.
I'm hoping it's heat or firmware. Power supply issues would be tougher to fix once it's out of the box and sitting in people's homes.