Hi,
Power supply is the CPU power from a 400W ATXv2
What is its 5V output rated at?
You need more than a PTC you need a real fuse, rated about 50% above the current drawn by your project.
As you have found a cycling PTC produces problems and if you have an overload will not permanently disconnect your project from the massive 5v supply you have.
It may have permanently damaged the device that is causing the overload.
I find CPU power supplies hopeless when it come to highly dynamic loads.
Also they are cheap and built for one thing, CPU and peripheral loads.
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Reverse engineer your project to draw your circuit, this way you will be drawing any wiring faults.
Thanks.. Tom.. 