Apparently the laptop that's currently functioning as the brain of my mame has trouble pushing enough power through the USB port to my minipac, resulting in irritatingly intermittent behaviour. I thought for sure it was a damaged cable, but I was a little surprised to find that the minipac works consistantly when hooked up to a desktop. The solution: a powered USB hub. I don't know whether this has been brought up before, but I figured I'd post it here in the hope that anybody who encounters the same problem in the future would be able to find it in the search.