Question : Problem: Have faulty motherboard or cpu; how do I test which it is?

I've been having issues with my music computer for a couple of months now and the faults are fairly consistent:
1) Screen flickering - almost always happens when using cpu intensive apps)
2) Total screen corruption on start up - this has happened 1/2 dozen times and is resolved by rebooting.
3) Abnormally high cpu usage when using cpu intensive apps - I used to be able to load heaps of VST instruments yet now it struggles when I only have a few going.

The tests i have done so far are:
1) Tried a different video card(brand new);
2) Removed all PCI devices;
3) Removed RAM and then reinstalled stick by stick.
4) Swapped out power supply;
5) Reinstalled Windows;
6) Checked cpu temps (was in the early to mid 40's C)  

None of these have fixed the issue so I am left believing it can only be either the cpu or the motherboard. The problem is I am currently on a very tight budget and cannot afford to fully upgrade my system. I also don't want to waste money on something that may not fix the problem(already did that with the video card).

My questions are:
1) does this behaviour generally signify a specific problem?
2) Is there some form of test software that may be able to determine which part is actually at fault? - I've tried some of the temp monitoring programs but all of them seem to show everything is ok.

Any advice will be much appreciated.
Cheers

Answer : Problem: Have faulty motherboard or cpu; how do I test which it is?

That the problem, you may be able to replace them.  Take the board to a good Computer shop or electronics shop to see if they will replace them.  This kind of board work is getting less frequent.  Basically if you can't replace them then you have to replace the board.

Good luck!