Your monitor wiring has 5 wires going to it from the game pcb - red, green, blue, ground, and composite sync. Depending on which monitor chassis you connect it to will depend on how you connect it.
That being said, I have two working Choplifter boards. One will sync perfectly with the sync connected to the negative sync pins. The other will sync perfectly with the sync connected to the positive sync pins. Both boards look exactly the same as far as I can tell, so I'm not sure what the difference is.
I also used an adaptor. If it was home-built, you should also check to make sure the sync is jumpered properly...