Galaxy S1 crashes overheats and freezes, battery level fluctuates?

[HPluv]

I've factory reset the phone, cleared the cash partition and formatted the SD card. What can I do to fix the below issues?

I was in the process of reinstalling core apps (nothing crazy, twitter facebook and songza) when the phone turned off (didn't even restart) so the above steps didn't help.

The issues:

-Top half of phone overheats within 5-10mins of use (not where the battery is)
- It freezes and crashes daily
- After a freeze battery always goes down to critical level even when fully charged. Upon plug in, battery indicates previous level though if I don't plug it in, the phone will turn itself off thinking it's got no battery!
- Sometimes it takes 20+ tries to open the messaging or phone app (it'll open for part of a second and crash, returning the user to the home screen)
- Media scanner takes forever even though the SD card is empty and very few apps on phone

The phone is out of warranty. I would like some home fix solutions I don't want to put money into this thing if I can avoid it. Also, I don't want to brick the phone or lock myself out.

Model: SGH T959W, Running 2.3.6 Gingerbread. There is no option to upgrade to Android 4.0 from phone. Can't buy an official Samsung battery from Canada and the ones on e-bay are sketchy.