A problem with the controller is almost certainly the cause of this as Heimdall says.
It could be the triggers or d-pad which can both be used to scroll up/down the menus. The triggers are particularly sensitive too, so you shouldn't rest your fingers on them.
But most likely it is the Left analogue stick. You probably don't notice it in games because your thumbs are resting on the sticks keeping them in the neutral position.
It was recommended in the early days of the PS(One) that the sticks were rotated just before boot up to ensure centring and some games even allowed you to recalibrate the centring in the menus.
In an older well used controller the culprit is often not just the centring springs losing their, erm, springiness but also 'finger cheese' ie. dried sweat, natural skin secretions from your hands and just plain old dirt and fluff. This can get iinto the moving plastic parts and cause problems.
If rotating the sticks as described and an extrernal clean up doesn't cure the trouble you may have more luck if you dismantle the controller and clean it from the inside out. Lubricating the the ball & socket part of the sticks with a plastic safe grease with PTFE (Teflon) ie. non-petroleum based can also help.
This post has been edited by fallenangle: Nov 9 2010, 02:55 PM