iPhone 4-Ios5.1 randomly reboots during use

Hi Guys,

Not sure if this is the right area, but based on the tests I have done, I'm guessing this could be a hardware issue.

Ok so basically, I have owned this phone since new, it's never been Jailbroken and the only "modification" that has ever been done is I've had the back glass replaced with a custom one I had made up.

The last 3 months or so, the phone during standard use tends to just reboot, the screen goes black, I get the little spinny wheel then it will either turn back onto the lock screen and behave, or it will continually do this.

I've also noticed that if it does automatically go back to the lock screen, some apps do not re-open, even after they have been closed properly from the home bar, they open then just close almost instantly which forces me to reboot the phone.

The worst case scenario is I have to do a full reboot - hold down home and power buttons till the screen goes black, then turn it back on, this can take upto 10 minutes sometimes which in my experiance is rather long..

Has any one else encountered this, or know of a possible solution?

The phone is outside of warranty, I'd rather not go and buy a new one if I can help it until Apple announce the launch of the next iPhone..

Thanks in Advance

Ry