Ok, I think I have a better handle on what's going on. Apparently the miners don't like ProHashing's coin profitability switching algorithm. Whenever it takes too long for a new mining pattern to get established, that's where it seems that my miner is having trouble, and just stopping. Since I added the c=digibyte field to my script, all three miners are working without issue.
I don't know if it's BFGMiner, or ProHashing's algo that's the cause, but apparently I wasted $50 on another hub, lol.