I had been having a problem for a while with my CPU overheating. Having built the computer maybe 3 years ago, I thought it might be due to have the thermal paste reapplied. So I got some thermal paste and thermal paste remover, reapplied to my CPU and heat sink. When I plugged the computer back in, and switched the power on, the only response I got was really strange. The computer would not power up. But the fans would all pulse over about a 3 second cycle, along with the LED lights.
I decided that I must have done something to screw up the CPU. Its an AMD Athlon and I know they are delicate, so maybe I accidentally damaged it while installing the heat sink, or something else along those lines. I procured another CPU off from eBay. Installed it. Exact same problem.
So previously I was sure I had a CPU problem. But now... could I have really screwed up the CPU installation twice? Could I have bought a bad CPU? Maybe I did something to the mobo without realizing? Its a strange symptom so I don't know what to make of it.

I'm having a similar problem of turn on for 3 secs/off for 3 secs/on for 3 secs, etc.
I checked my PSU and RAM and HDD and basically everything except the CPU and mobo, and everything else worked fine. I then replaced the mobo and it did the turn on/off thing once and then proceeded to boot right up. Strange that every time I turn it on it will do the on/off cycle, and I'm still trying to get answers, so if you have found any please let me know, but in the meantime I would suggest that you replace the mobo and see if that fixes your problem. It's possible that you damaged it when working replacing the CPU's. Before you do that you should of course reset the CMOS (just remove the battery for a couple mins) although I doubt that will work for you as it did nothing for me.
Best of luck!

Pull the memory sticks out and put them in again. Clean their gold contacts with a pencil eraser. Test it one a time in different slots.

Reset your cmos memory, refer to your motherboard manual.

Check all the power connections from the power supply to the motherboard. Test your system with a known good power supply.

Hope this helps...

ho, check your psu it could be a dying psu..

Any easy way to check the psu?

hi pluskin, short the green wire to any black wire, this will power on your PSU..and check the voltages... the red wire should be +5 V, orange +3.3 V, Yellow +12 V, check the web for the complete output voltage of the ATX power supply

I finally found the problem. I feel pretty stupid, but all this time, the power supply plug to the motherboard was loose. I must have partially pulled it out while working on the CPU. I was planning on testing the PSU, and when I saw how easily it pulled out I figured I knew what the problem was.

Thanks for the suggestions.