When it goes down try pinching the EOS briefly with your fingers. I had this problem in the past with data east games and it turned out on one that the EOS needed cleaning and another that one of the contacts was loose on the leaf. Also seen cracked leafs exhibit this behavior.

If you lift the right flipper up with your finger and hold the button in does it stay up with the coin door open? This might tell you if the symptom is a red herring.

Have you checked to make sure the leaf switch is disengaging?