A PC only needs one stick of any type of motherboard supported, functional ram to pass post. Open up the case and check that the slots are not dirty, and then reseat them firmly. If the problem still persists, try swapping them. Also check the ram memory number shown when the system first starts up, too.

Try taking out the suspect stick and see if things improve. If they don't, then it's not the RAM at fault.

Could be something wrong with the PSU, or when you were cleaning parts and taking them out you could've bumped/broken something. But yeah, take out the ram that you had problems with.

if you took the heatsink off of your processor, you will probably want some more thermal grease.

I am willing to bet that your processor was overheating and shutting your computer down.

Hahahaha, that stuff is very important in the process of transferring heat off the CPU, you're probably overheating.

You can order some of just about any PC equipment website. I don't know what ships to FIJI or wherever you live though.

I'd suggest getting Arctic Silver 5 or whatever they are at now, it seems to be the preferred choice. Ultimately it's a REALLY cheap way of prolonging PC life and increasing system stability, good thermal paste makes a hearty difference!

You just smear it on the bottom of your heatsink, over the entire area that contacts with the CPU itself.

How to install Arctic Silver thermal solution:
First of all, ground yourself, then rub off the remnants of the last thermal solution from the top of the processor and the bottom of the heat sink, preferrably with a dry paper towel. Then you dab a BB sized amount on top of the processor and you flatten it out evenly with a credit card. Try not to touch the surface of the processor with your fingers, and make sure no bubbles, hairs, or other foreign particles are in it. You then put a grain sized amount on the part of the heat sink that lays on top of the processor and rub it into a processor-sized circle to make sure it gets into the micro-chasms (I just use a plastic bag over my finger). After that, carefully place the processor back into the socket, and the heat sink back on top of it. Make sure the fan is still plugged in and fire up the system. You'll want to leave your system running for about 48-72 hours straight to allow for the heat transfer particles to settle.