macbook mouse track pad sensitivity

I finally moved from Windows to OS X several years ago and have been very pleased --except for one *very* annoying problem. The MacBook cursor randomly jumps around the screen when I type. This happens in all applications where I'm typing. I will suddenly realize that I'm typing 3 lines above or, in the case of email, even in the address box! I have read where this can happen if your thumb touches the track pad, but it happens even when I have an external mouse connected. I see no way to adjust the sensitivity of the mouse track pad.

I'm using OS X 10.6.8 with all updates, but I also had this problem with earlier versions. I'm wondering if anyone else has this problem and if there is a solution?

Thanks for your help.