I did just notice though my left hand rest right above the alt key. I've been thrown back a page once in a while so it might be just letting your hand barely hit the alt key.
After typing in a long post or using programs like excel for example, your hands tire out and make their way towand a more natural resting position bumping the alt key.
If that's not the case for you then take the keyboard apart and clean it. Sometimes small amounts of corrosion will act like a capacitor and fire randomly. However it would be quicker just to score a usb keyboard.