I need help with my computer! It will really be needed from a very knowledgeable tech guy, please help!?

Okay I have an hp slimline s3500z (AMD 4850e 2.5 dual core, 4gigs of 800mh ram, 500gig hitachi hard drive, m2n61-ar mobo, windows 7 ultimate 64bit) and I've heavily modified the power supply and video card. It originally had an 160watt power supply, which i upgraded to a 300watt power supply. The video card it had to begin with was an nvidia 9300gso 128mg, I upgraded to a 9600gt 512mg. When i first did these mods it ran my games beautifully and everything was much faster, but for the last month or so randomly all animation would freeze and all i could do was restart it and it would just restart fine, but the past two weeks what would happen was it would freeze at the windows loading screen and i would just have to restart it again, and again, and again, and again. Literally before i could even access the home screen i just had to restart it around 30 times, I know that its probably something to do with the pci-e bus on the mother board, because its a 1.0 and the card is a 2.0, at least thats what i'm guessing, so before i have to spend the 100 dollars on a new atx case and mother board I'm hoping some one could help me out with any kind of quick fixes. Any answer is GREATLY appreciated.