Score resets at 1000

[mcgrailm]

I recently picked up a Wonder Wizard demolition derby pinball machine and it was working fine and now the score resets every time it hits 1000 the score goes back to zero and then only the bumpers only add to the score. If I let the game sit for a while it works fine for about three four games then it starts acting up again.

Does anyone know what may be causing this and what I can do to fix it ?


thanks
Mike