Random cursor jumps

ASUS laptop, Windows 7 Home Edition, Firefox browser. Every so often, when I'm typing text in a Comment box on a web site, when I pause to think and start typing again, the cursor jumps backwards through the text - often a few/several lines, within the earlier line of text, and I'm suddenly typing within a word. For example, I might start this sentence, and suddenly be typing 'For' within the word 'box' in the second line of text above. It's been going on since I've had this laptop and it's annoying, but I've never been able to figure out what was causing it. I've even tried banging random keys and the touchepad to try to replicate it, but it only happens when I don't want it to. I know there are keyboard controls that move the cursor to the beginning of the line, etc, but this is different - the cursor goes backwards in what seems like a random manner.

Sound familiar?

Asus makes good stuff. Hmm.

If this is laptop, be sure you aren't touching the ball of your hand (below the thumb) on the trackpad while typing. It's a common problem.

It is a laptop, and I've tried to cause the problem by touching the trackpad - never could do it. I'll try some more.