When I turn on my PC the fans come on as normal but it wont even POST. It does this for anything between 5 and 50 tries, until it suddenly decides to work. I have tried everything: resetting CMOS, replacing it, unplugging every component and plugging it back in etc. I have heard it can be due to bad RAM, but I did a 15hr memtest with no errors, and even tried some different RAM, and it still does it. Anyone know what it could be?

have you added any new hardware lately?

Nope

Try unplugging everything, hd, cd, floppy...ect, and see if you get post then. If so, reconnect devices 1x1 to see which one may be causing the problem. If not, do you have a multimeter to check the output of the power supply? The 4 wire plugs going to the devices with 2black, 1red, and 1yel, are the power source. Turn the pc on, and from yellow to black should be 12volts, red to black, 5 volts. If no power, try unplugging the power cord from the back of the pc that goes into the power supply, leave it unplugged for a few minutes, and while it is unhooked, hit the start button a few times. Some power supplies have a thermal overload that shuts it down if it gets too hot or overloaded, this may reset it.