A bad CPU fan will cause this behavior with some motherboards. Others just let the CPU fry. Power supplies can, indeed, cause these symptoms as well, as can a flakey ATX mother board. If it's AT, not ATX, it's definitely the power supply.

I use Super Power cases and power supplies. The cases are sturdy (a bit on the heavy side) and open with a single thumb screw. The power supplies have been consistently reliable. Power Supply problems are a real pain and I like to avoid them.

I am very fussy about cabinets on issues of ease of maintenance. I will be maintaining and upgrading the machines I build for many years. If the customer didn't want me doing the maintenance, they'd have bought something cheaper elsewhere.

I recently got an order for a desktop machine and cringed. While I still maintain some desktops I haven't built one for quite a few years because they're so difficult to work with. Nontheless, I ordered a Super Power desktop case and have to say this, they make the best desktop case ever made. Even easier to work with than the mini-towers.

. . though ATX was also available. Just look on the back of the cabinet. If there's a removable plate including the keyboard, mouse, serial, parallel etc. ports, it's ATX. If there is no insert plate or there is one with only the keyboard and all ports are in slots, it's AT.

Of course, there's probably some mixed ones. The AT motherboard we used for the last year we used AT accepted either AT or ATX power supplies.

With ATX, the power on/off is handled by the motherboard and the switch is momentary. With AT, power on/off is handled by a real on/off switch. Microsoft found this unacceptable because Windows didn't have control over power on/off.