Facing this problem I was really frustrated. I tried all methods like rooting, using custom kernel etc. Then it came to my mind that all this started to happen after I did official update to 2.3.3. For about 1 year i couldnt figure it out. Even if surrounding temperature increased the phone would reboot. this situation was really ridiculous. Tried and tested in many conditions I found out that after flashing to the firmware that the handset came first from factory ie 2.1update there is no problem of heating up or rebooting. I agree many functionality are not there in 2.1 when compared to 2.3.3. but u can be rest assured that ur phone wont get heated up and reboot while using 3G.
Tried on 3 different Xperia X10i 's the problem got solved...... If u really want to use X10i with 2.3.3 then its possible only if u can keep the phone cool by having a dedicated table fan or AC blower directly blowing on to ur xperia X10i while on 3G Connection