Probably not enough power then. Does it only initialise 5 chips if it can't power up all 6?
The usual reason for an older version working better would be: sheer coincidence and nothing to do with cgminer. Though of course any bug is possible, however I'm not seeing the instability you speak of. The only change in the bxf driver between those versions was making bxf bits configurable on 4.2.3. What OS are you running it on? Are you giving it any extra options, bxf or icarus?
Probably is the power. I've given my hex 6A and its rock solid for hours at ~13 (stock speed).