There are only couple of things that you haven't replaced, the PSU, and the HDD.
Can you confirm that there is nothing overclocked, it's all default.
And this is on any and every game you play ?.
Can you download this piece of software and use the benchmark part, you can test your system to see if and where the problem is. SiSoftware Sandra - Free software downloads and software reviews - CNET Download.com
I'm guessing you have all the latest drivers, GPU, Sound, Chipset, and all the rest.
When did this start, have you installed/un-installed any software/hardware. (other than the new parts fitted).
What PSU do you have at the moment ?.
Try the bench-testing software and post you results, good luck.