"zig-zag" lines? Need more info.
I can't reproduce that, on a G4 AlBook with Radeon 9600 running 10.3.8, Xcode MacMAME 0.95 with the GL plugin.
But, there are two possible causes for intermittent artifacts:
1) you turned off VBL sync. This will cause horizontal "tears" across the entire screen. Some people have had the GL plugin tear even when VBL sync is turned on, but I can't reproduce that. It might be caused by other apps they are running in the background.
2) timing issue with some video cards can cause texture corruption such as in
this pic . I haven't seen this in MacMAME for a long time though.