I had this same problem. My machine had a free play button that I had removed before purchasing without bothering to consider why it was there.
My board is the one with the same scene for all higher levels (the one of the championship). It is not a dip setting. I think its a flaw in the design. Rather than simply showing a free credit, it seems to just activate the credit while in two player mode. If you coin op a bunch of credits in regular (non free play) mode, it does the same thing. One player is fine, but two player will run untill all credits are used up.
The only fix was for me to re-install the free play button (can be wired to the coin mech). I'll be glad to post a pic if will help you. I used a less attractive radio shack button and drilled through the coin door because it was thin.
The button can be used to enter one credit at a time as needed.
Good luck