Hi, I'm having exactly the same problems on my router, I've got a 2160 with extended i/o running off an intel XP pro box with little else but Mach 3.042.40 installed.
It hangs in the middle of a program, spindle running, as though it's a feedhold, all servo drives enabled, sometimes Mach estops, sometimes just seems to hang, always needs a restart (otherwise next program run fails to start spindle). It can run a file for several hours before doing this, or sometimes 10 mins.
Get "Galil lost connection" message usually. As mentioned by capsteve earlier (too many steves in this thread already!), thought all our previous comms dropout problems had been sorted when we found a broken screen in an encoder cable (noise, apparently), but still happening, albeit much less frequently. Going to reinstall on a fresh PC tomorrow.
Will try the 'start spindle, cylce start' workaround tomorrow too. Had the odd little hiccup which might shed light on it, such as today when homing, one axis just ran over both the home and limit switches, then when almost beyond the back end of the switch ramps, suddenly Mach lit both LEDs, as though it had dozed off for a second..
Battwell has done all my Mach setup, so I'm sure he'll be asking much more in-depth questions, but I just saw this thread tonight after a frustrating day and thought I'd pitch in!