Glitch

basically, it seems for a lot of people the GPS packs up after about half an hour from booting. Reboot gets it back again, but then about half an hour later it goes again.
Might be worth waiting if you were thinking of buying.

overheating problems? They get very hot in all devices anyway, so poor cooling could cause problems.

I have noticed that my iPhone gets hot when I turn on the local hotspot, literally. Not really noticed the problem with GPS on.

I think it's somewhat unlikely as a reboot wouldn't drop the temperature that much immediately, so you'd expect the periods for which it worked to reduce each time you rebooted it, which doesn't appear to be happening. It seems more likely to me it's a memory or file system code leak which is causing something in the GPS hardware driver to crash roughly half an hour after the GPS process is started.

Also wouldn't most chips be designed to cope with high temperatures. I would imagine that it would take 30 minutes or so for the temperature to drop to normal levels if it were off, so rebooting would mean that the temperature would be still too high if that really were the cause. I agree with you it is a software problem. It seems like an error of some sort that seems to accumulate over time till it overflows and crashes. So depending on the starting position it might reach the trigger point quicker.