Phone restarting or freezes whenever it wants. Any suggestions?

I do not remember exactly, if it has started doing that after I have dropped it to the tough surface or before. However, phone restarting or freezes whenever it wants. Mostly when phone getting charged, it is next to me (no matter if it charges wireless or wired) and I observed it restarting or freezing more frequently when it is charging.

Freezing in this instance means that screen goes black, as if it would be turned off and phone does not respond to any button or screen commands. You cannot turn it on by pressing power button and only restart with power and volume button turns it on.

Might be helpful. Phone has some visual defects after aforesaid drop:

1. Dent on the top backside (speaker side) - http://rghost.net/7qS7qD46w & http://rghost.net/7bYtzyK9m

2. Surface of the screen at the top where the "Nokia" writing is, is little higher than aluminum side facing - http://rghost.net/6lK9GhdQw

I had proximity sensor problem as well as all above and official service could not help me (I think they are just useless in my country) reporting that they have replaced the proximity sensor (which is still a problem).

Any suggestions how to fix it?