First check that the pop bumper switch is adjusted properly and not set too close.

If the switch is properly adjusted, then it could be a faulty disc capacitor on the pop bumper switch.

You can try removing this capacitor to see if the problem is rectified.

Check section 3j in the first bally section of these guides for more info.
http://users.esc.net.au/~boots/Pinball_Guides/default.html