Sounds like you're dropping below 4.9 volts on the cpu (at least that's what my problem was). Check your voltage on the cpu and see where you're at. If it's happening with a new driver board installed, I'd look at the "Z"connector that powers the auxiliary driver board. It's a known weak link on TZ, IJ and STTNG. I've had to get rid of it on each of those games. Check all your other connectors in the power chain while you're in there for cold joints/cracked pads on the board and/or burnt connectors. Re-pin/reflow as needed. There's other potential causes, but I'd look at these first if your working with a driver board you know to be good.

TZ is really no different then any other WPC era game. There's just more stuff in there. Once it's tuned properly, it's just about bulletproof in a home enviornment, as are all the B/W games from this time period (at least in my experience - your mileage may vary).