Raleigh wrote:
Thank you.
I haven't added anything new, except for a new motherboard.
It's completely powering off, then restarting in about 5 seconds. Repeatedly.
As for age, that's tricky.
It's been rebuilt a few times.
If I take out the power cord from the back, wait a while and plug it back in it seems to stay on longer.
I've tried unplugging a few things, I'll see how it goes.
Glad you've sobered up

Possibilities,
1. Bad power supply, replace power supply and see if problem persist
2. I suppose the power cord itself could be bad, do you have another one?
3. Inadequate CPU cooling, check temperatures, ideally you want under 80c, preferably under 60c
4. Inadequate GPU cooling, check temperatures, ideally you want under 100c, preferably under 80c
5. Bad video card though they normally cause artifacts on the screen, is anything graphics intense running?
6. Bad Ram, run memtest86+
7. Bad Motherboard though unlikely cause they tend to not boot when bad
8. Also unlikely but your outlet could be providing dirty power, a battery backup can solve this