Why is my phone painfully slow? Hardware issue

Hello!

I have question regarding my HTC One M8 Harman Kardon, I sent it to a repair shop because it suddenly slowed down incredibly. Like imagine a phone running on 2 Frames per second. It happened out of the blue, i never dropped the phone, it has zero scratches its basically new. No damage at all. Tried factory reset, dalvik cache cleanup etc. but when i took it to a service they told me something is wrong with a CPU (they didnt open the phone). It doesnt receive enough power, but when you twist the phone lightly holding it from one corner to another, it speeds up again and works normally. Seems to me there is simply a bad connection with the CPU, is this possible to repair? Can it be done without replacing parts or should I get it replaced? Is this even possible to replace? I mean the part where the processor sits. I bought the phone second hand and it had S-OFF unfortunately so I cant claim any guarantee. I tried going to developer options and turning super CPU maximum perfomance boost but it doesnt change anything.