Strange behaviour ...

I am using a 2 year old MacBook
Model Name: MacBook
Model Identifier: MacBook4,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.4 GHz
with Snow Leopard updated to the most recent version. I use an external Apple wireless keyboard.

Recently I began getting very odd behaviour. When in Mail, emails delete themselves as soon as I click on them and text input is erased as soon as it is input. In any browser (Safari, Chrome or Firefox) the browser goes to a new link for a second then goes back to the original one, no staying put in the new link -- just flashing right back. In other programs, like Word, text disappears while still typing. There is a sound, the same sound as happens when an input error is made, which taps out on an irregular basis.

This is not only annoying but makes using the laptop all but impossible when it is happening. It does not happen all of the time. It seems to stop when I turn off Time Machine (I use a Time Capsule). I have run Spring Cleaning to clear out caches and repair permissions and Sophos Anti-Virus just to make sure. I have yet to do a total reinstall of the system as my Snow Leopard disc seems to have been misplaced in my most recent move.

Anyone have any idea what might be going on and how to fix it?

Thanks!
Susan
dearvicar -- nowhere close to a tech