low battery? try remove battery and then put back in and charge and turn on

What ROM are you on? stock 2.3.3 from SE? 

Might as well u 'repair' the phone with SEUS or PC Companion or flash the 2.3.3 firmware with flashtool. Fresher 

Since factory reset restore to stock condition.

Also the lag could be because of the FPS Cap? why not try using custom ROM like T.E.A.M. ROM ? It's fast 

Yes you can, just use flashtool and flash the global generic or cust-MY(Malaysia) firmware file. It'll be unrooted as well.

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Added on February 13, 2012, 9:25 pm
Support Zone?

Ouch well there's another to fix the lag on stock ROM.

Have you root and installed xRecovery on your stock 2.3.3?

Download FPS Uncap: http://forum.xda-developers.com/showthread...light=fps+uncap
and
Download Audio Lag Fix: http://forum.xda-developers.com/showthread...light=audio+fix

Put them into your MicroSD card, then boot into xRecovery(tap back button few times when you see the 'SONY ERICSSON' splashscreen) then  wipe cache and go to advanced>wipe dalvik cache, then Install custom.zip and select the 2 files and flash it, then reboot and you'll have a smooth stock 2.3.3 ROM

choose zergRush.. sorry haven't really updated the guide.. but the rest you should understand:D

T.E.A.M. ROM already has both.. but if u still feel FPS is capped (check with FPS test from market) then flash the FPS Uncap..

In my case I don't have to though, but my friend's had to flash it.. maybe the installation didn't went smoothly..

Remember to factory reset, wipe cache and dalvik cache before you flash the 3.8 ROM and 3.8>4.0 update