Exclamation Strange Problem with Battery?!

Hello there,

A few weeks ago my galaxy S2 started have a strange behavior..It suddenly started getting hot so much that i felt it in my pocket as it was burning..That effect lasted 2-3 days and then it died..

I Ordered a new battery couple of days ago and the phone seems resurrected...Booted up without any problem and worked fine for 2 days.

Then today it died again..I tried to open (should have a 90% of battery by that moment cause it was updated to a fresh rom a couple of hours ago) it and i felt it hot, then i realized that it was dead again..Not booting up,not charging,...

Seems that the problem is that those batteries died, cause i tested them in a friends s2 and they are not working there,too..But with his battery my phone boots again?!??!

Any thoughts where is the problem and if its worth fixing it?