I have a MacBook that was exhibiting a startup-beep with three repeating short beeps, which a search indicated is caused by RAM issues. It's an older machine running Snow Leopard with 1GB of RAM (awful, I know, but this was a budget college machine and I've since upgraded to 4GB on a newer MBP). I re-seated the RAM sticks, same issue. Tested each stick in each slot, and the machine starts up fine with only one stick installed. Both slots work with both sticks, just only one at a time. The pair won't work, no matter how I switch them up. What could this be caused by?