Random crash and reboot when warm.

Currently, my iphone 3gs will shut down and reboot almost daily under moderate to heavy use (games, photo sharing and editing), but good under light work (can read an ebook for hours).

It's 2 years old and has been dropped 5 times in the first 6 months. Symptoms started about 6 months ago, but less frequently so they don't seem related to the drops.

The first crash happened while on the car charger with a nav app, then a few weeks later with a different nav app, had to restore through itune and got error 1603 so I thought it was the gps.

Then it happened with music with the car charger, and one time it crashed the second I unhooked it from the car, so I thought the charger was bad.

Eventually it started happening while playing games. Even under the sun, it can crash. Then it would go into a constant reboot loop showing the apple logo, and sometimes the 'plug into itunes'. It went through a few restores via itunes, sometimes an error and sometimes not, until recently it would sometimes crash halfway through a restore. Now, I risk a crash if it's in my pocket too long.

It took of while of troubleshooting, but I found that if I put it in the fridge for a short time to cool it down, it will turn on in about a minute, even if if I get the itunes logo. I've also tried using it under heavy load with an ice pack on the back of it and it seems to work fine, and would restore perfectly while sitting on the ice pack. I'm sure it's heat related. My friends have never heard someone say, "oops, almost forgot my phone in your fridge" ;)

I took it apart and it was loaded full of 2 years worth of pocket lint. I blew it out with canned air and all the contacts look fine, went back together and still works, but no different.

Can this be a battery or cpu problem?

Appreciate any help.