Curious crashing behaviour in iPhone 5

iPhone 5.

Background:

Had been sent to 3rd party repair shop for power button repair. Harmful modifications may have been made.

Battery had been replaced before.

Phone had been self-opened to fix a certain problem. Help courtesy of iFixit.

Above operations were carried out at least a month before crashes started occurring. Most probably not the direct cause.

Symptoms:

Crashes cause screen to half its brightness or develop bands of colours. Phone would be unresponsive, requiring a restart. Phone may restart itself automatically sometimes.

Cause:

Crashes occur if i pinch the top of my phone, ie press the glass into the alum case. If care is taken to avoid the cause phone will not restart randomly.

I suspect a short circuit somewhere at the top of the phone, but cannot verify.

1) Would anyone know the possible locations of a short circuit?

2) Can anyone propose an alternative cause?

Thanks a lot.