GPS reboots the phone When I use a GPS-intensive app (like Sport tracker or endomondo), if I turn off the screen, the phone reboots within one or two minutes.

What is the problem? What can I do?

How can I check the temperature to know if it's overheating? What can I do in this case?

(My phone is a Nexus One on Videotron (Canada))
Overheating.

You may have used it intensely and when you lock the screen you then put it in ur pocket?

Your phone will be overheating and restarting itself to stop damge to the circuits/processor.

Similar user issue here