Sadly, it's not very stable (I suspect our null modem 'emulation').
There are inherent timing issues with importing and exporting serial data from an emulator. There is a one character time of latency as a minimum and no timing is maintained between characters. Modems also cause the same problem for real hardware & these problems are not solvable.
It should be easily possible to log out the data written to the uart, log out the data sent by the "null_modem", log out the data received by the "null_modem" and the data received by the uart and compare that what the game sends on one side is received by the other (and vice versa). If it's not then it's either the null_modem code or the uart send/receive code. If it is the same then the problem is somewhere else (including the original game if it doesn't work reliably with modems).
If the game was unreliable on a modem then it will only become reliable when both computers are emulated within the same running_machine/scheduler.
