Further testing seems to show that it's not really a boulder problem. The game has just gotten generally unstable after the appearance of the GC. I've tried keep him away from the boulder, like parking him at the research table and I'm still getting errors. Sometimes as soon as I load it. I'm going to try letting it run for a while without watching to see what happens. The cave was the last puzzle so I guess I'm through with this game anyway, but I just wanted to see what, if anything, happened.