..have you checked the temperature that your system is running at? This can often cause the problem you describe.
If you go into the BIOS (press del at start up) you should find the temperatures. Are all the fans running and are they free of dust and other gunge, particularly the video card fan if present
SisoftSandra will tell you temperatures and fan speeds
To answer the question you asked I recently bought a standard ATX case with 420w PSU for under £30 from Dabs. It was well made and easy to work on. It also had a rear case fan fitted