non-mechanical key failure in laptop

I have a laptop (dell inspiron 8200) that exhibits a problem.

Sometimes the left Alt key doesn't work, however sometimes the OS thinks the key is being held down (while the physical key position has no effect). And sometimes (very rarely) the key works fine.

I firmly believe the problem is not due to the mechanical switch in the keyboard, because when it DOES work, it works so well, and the physical condition of the key is fine.

I reinstalled Windows XP and updated my CMOS to eliminate that as a possible cause, and the problem still happens. Plugging an external usb keyboard works fine, while the internal keyboard ALT key still has no effect.

Any ideas on how to remedy this problem?

Well 2 pieces of plastic/rubber is still mechanical (or physical), as opposed to electronic. The reason I doubt that it's mechanical is because when it works, it works perfectly, but then suddenly gets into a mode where the ALT key just doesn't work at all, which apparently has no relation to a physical impetus (and sometimes it's stuck in an always on state).

I really think it has something to do with the electronic chip that the keyboard plugs into, because the keyboard is not affected by any physical impetus at all.

OR if the "scissor" mechanisme and landing pad is free, then you should unmount and detach your keyboard and use some contact cleaner / lubricant on the conector and reattach. See if that works.