Both machines are core i7 64 running Win7 64. Both use a bunch of erupters and one has a BFL LS as well. At first I thought the instability was caused by the heavier load on one machine but now I'm pretty sure it is not load-dependent.
A Raspberry Pi would run ALL your stuff - load has got nothing to do with it, as there isn't any on an i7.
Shit, I have 30GH/s gong through a TPLink 703N (a £10 router powered off USB with a Dorito for a CPU and 8MB RAM).