I received a mega-order from GSE-Reactive today and immediately put it to use. The TWGS board got a new set of GALs, the 32KB cache board with 1.8S firmware, a new 65816, and various crystals. You might also see that I pulled the VOC for now and put the Uthernet in Slot 1 (with which I have not yet had time to mess).
So far it's very stable at 7MHz, but temperamental even at 13.75MHz, and will crash before it even boots at 18MHz. Changing out the PSU with another (curiously differently-labelled) one seemed to help somewhat. Other factors that are known to help are reseating the oscillator, cold air, and reseating the CPU cable and ZIF socket. Yeah, I know filtered power and a heavy duty power supply will help tons as well, that's on my list.
There are many more variables I can play with. I have a few ROM 01 and ROM 03 boards here, each with its own tolerances and quirks, and one will probably clearly prove itself to be more stable than others. Also note that the board I've been using was never intended to be sold to customers in the first place. So I'm not worried, but it's going to take me a few days to get it up to rip-roaring speed along with everything else.
So give me another GS-messin session or two, and I'll post about my stable IIGS and give my impressions of the Uthernet card too.
-Chris