Your problem is power-supply related. You are using an ATX power supply, which by itself is a huge no-go, as they normally don't have the 5V rail as their main power rail. This means you have high ripple on the 5V line, along with poor regulation, because the main 12V power rail is under-loaded.
We did solve quite a few problems with A500+Indi-ECS power issues with a patch on Indivision ECS. Please contact your reseller to find out if your unit is already patched (been delivering them with the patch for a good two years now), or if it may still need that patch.