This is a bug with interlaced modes on recent GM. Instead of setting syncrefresh 0, you can enable frame delay from the ui and increase it until it gets fixed. But you need to do it per game.
Ah okay thank you, I am glad to see it wasn't something I did.
In the mame.ini what is the difference between monitor generic_15 and monitor k7000?
Which one am I better off using?
edit: the answer to my question is monitor should always be set to "custom".