This is a Dell Optiplex GX620. When I hit the power button, it alternately turns on for a few seconds, then off for a few seconds, then on for a few seconds, and keeps going the same way, but never starts. I tried using a power supply that I know works. The capacitors on the motherboard are good, at least to appearance (none are swollen or cracked open or leaking) Have you ever run across this behavior before?
More about :turns seconds seconds seconds power supplies
only time my computer failed to boot up like this was when i plugged in a fault 6 pin into the graphics card, try taking out the graphics card or HDD or SSD seeing if that solves the issue just to make sure they aint loose and causing a small issue..
Yep, I've had this a few times... Have you added any new hardware? It could be a couple of things, in order: motherboard, psu, CPU. Since you have tried the psu, next thing would be to remove all but one stick of ram, remove video card and everything else that doesn't need to be there, then try start it up. If it works, then add the next component and try again.
Maybe re-seating you ram and power connections etc will help! If not, it's most likely your motherboard... Power surge or something may have caused some damage.
I have had this problem twice, once it was the motherboard, and once it was the psu.