2 possibilities, one your internal components are gettin worn or dirty (although I though the MS sticks were optical? may need cleaning), two perhaps there is a problem with the soundcard joystick port.
Didnt yours come with a USB adapter? Try it and see if you still have spikes. If so, its the joystick port/soundcard. If not, you need to take your joystick apart and clean it.
This is of course, ruling out bad drivers and/or other software that might interfere?