The problem is that usually even just having a debug build that would allow to 'look into things' more carefully already changes behavior.
I haven't tried that yet with the current status, but this is the common 'experience' with such issues (i will still try though).
What i already did in the past is to use static code analyzers and runtime memory checkers on that core, but both with no substantial outcome. :/
For comparison: We also had an issue with 64bit builds and Capcom emulation. In february i 'fixed' this by restructuring some code.
So no real fix, just trying to clean up fishy stuff, but that already did the trick due to some sideeffect.
At some point this may haunt us again, but as there was also nothing i could find while debugging, this was the best 'solution'.
not ok:
Mexico 86 (maybe bad ROM dump ? - but vpinmame checkroms says all ok, both mexico and mexicofp)
Thanks for the tests!
Mexico 86 is simply not finished up yet, as a lot of missing pieces for complete sound emulation are still missing.