Very big help needed

[Jarm__]

Dear members!

So I have come to you all yet again for more help.
My problem is the following: The computer turns itself off. For example I am playing, or browsing the internet and the computer just magically randomly shuts itself off.
My build:
Core 2 duo e4700 @ 2.6 GHz
3 GB DDR2 Ram
Gigabyte HD 6850 1 GB
Asrock 775i945GZ Motherboard
500W PSU

Here are some stuff I need to mention;
It all began with my CPU Overheating (hitting 90-120'C) So, I got a new heat sink and reapplied thermal paste, and ever since that this has been happening. During gaming, or web browsing, music and all general usage. It just decides to turn itself off without warning. There are also times when the computer works perfectly. I have tested by taking out the GPU, and it works flawlessly with a 450W PSU, so I decided to replace it with a 500W PSU and I putted back the GPU and, yet the computer decides to turn off, during gaming...sometimes it doesn't. I am aware that my computer components aren't overheating (Gpu went max 55 degrees, and my CPU is at 45 at max). Another thing to mention is that, if I have my computer turned on, and I decide to plug in a USB at the back, or remove anything, hell even touch the computer case, It shuts off.
It's very annoying now, and I'd like you all to help me with some ideas of this matter. I appreciate all the help that will be given.

 

[Ilikecomputersandicannotlie]

check the fan and see if it applies too much force on the cpu. check if any thermal paste got in the cpu pins, and clean it out. this sounds like a toasted vrm issue, which you need to get a new mobo for. make sure your power supply connectors are tight and if the power suplly wasnt tested, do it now.

 

[Jarm__]

So, the VRM isthe issue you say, apart from getting a now mobo anyway I could fix it?

 

[Insomniac Jack]

Sounds like something is causing a short and grounding to your case. I'd do what the post above said but also check for any loose nuts, screws, or anything metal under, around, or touching your motherboard.