Three long beeps at startup is the error code for an improperly seated add-in card. Crack open the case, and firmly press down on each and every PCI/AGP card in the system. Odds are real good you'll feel one of them slide into place just a little bit, and that will be the end of your problems.
And if you want the detailed technical answer as to what's happening. As your computer warms up, the parts inside start to expand. This makes for just enough of a gap that the card pops out of the slot far enough to cause problems. When it reboots suddenly, it's likely do to a bit of current jumping to the wrong connection point and causing a failsafe to kick in to prevent damage.
One thing to note, is that heat buildup is another major cause of random reboots. So if the rebooting persists, try running the system with the case off to see if the increased air flow does anything.
i have a emachines t5026 with upgradeed pci graphics card. anyways recently ive been having problems with random restarts. also ive been having problems starting the computer up. when i start up, there would be 3 long beeps and the screen wont turn on, keyboard lights are off and it just basically stalls. please help me find the cause of this problem. thnks very much