sadly this is a hardware issue... Whenever the phone warms up a little bit.
I didn't have that issue in the first months but ever since then and more and more frequently. Right now I'm fighting with o2 to get a new device, because this doesn't seem to be repaired.
Do the following test to make sure you have that hardware-bug:
Let the battery charge for a while. When you notice your phone getting a little warm, open a few applications (google maps, navigation, mediascape) whatever... The phone should immediately restart itself.
When you remove the battery cover it might also smell a little burnt.
Congrats... You're affected and it's only gonna get worse...