Computer won't get to POST screen, turns off, restarts, rinse, repeat

I am having a problem with my desktop right now, where I turn on my computer and turns itself suddenly, stays off for a few seconds, and then turns itself on again, only to turn itself off again, and this process keeps repeating itself until I force a hard shutdown. However, when I look inside the case while this is happening, I can see the fans spinning and LEDs lit just fine, business as usual.

Like the topic says, my computer does not reach/display the POST screen during this whole time.

There are a few suggestions (according to a friend of mine) as to what is going wrong with my computer and they are the following:

Your PSU is going bad.

I would think this is the case because I keep my computer running 24/7. The only time I would actually shut it down is when I am away for a weekend or longer. Plus the power supply I currently have is almost 3 years old, since I bought it in summer 2010.

The CPU, heatsink, RAM, connections need to be reseated/reconnected.

I have done all these yesterday and the problem still persists. I even made sure to ground myself as I was handling the hardware.

Anyways, do you guys have any other suggestions as to what's wrong with my computer? I need more information before I make any further attempts to fix it.

sounds like an overheating problem to me, have you cleaned your components recently?

I clean the inside of my PC once a month, and this month's cleaning was a week before this started happening.

Try starting it without ram, do you get the correct warning beeps?

Try maybe borrowing a cpu and see if the problem still persists.

Take out the graphics card and see if the problem continues.

Just try to narrow it down to the correct component.

Unfortunately, the motherboard speaker that came with my case does not work anymore, so I really can't answer this. I'm going to get a new one soon, though. However, I did try testing out all four sticks of RAM, seating them one at a time for each bootup. The problem still persisted through all four of the sticks.

I haven't tried using a different CPU to find out the problem, as I do not have any spare CPUs, nor do I know anyone that has spare CPUs.

I did at one point remove the graphics card and relied on the integrated chip and the problem still persists.

I had a pc that did the same and I just let it sit for a couple days and it usually would turn back on. IDK why it did that because i swapped the PSU, RAM and HDD. So I think it was the mobo going out. I didn't have another CPU so yeah. So after a while it wouldn't even turn on then it just died. RIP my first PC.