I'm not familiar with the Biocube specifically. I can try to help generally though...
The fan in these systems always runs off a separate power supply, so the fan running has nothing to do with the ballast.
I assume the lamps... the actinic and the daylight... are the same type of lamp. If you reverse the position of the lamps, does the problem follow the lamp or stay with the suspect socket?
If it stays with the socket, the first thing you need to look at are pretty basic... check the contacts of the socket, make sure there is no corrosion or anything like that. If there is, try to clean it out, of course with the lighting system disconnected from any power sources.
I would expect that there is a set of inline plugs and sockets between the ballast and the hood. Have you made sure that those connections are tight and mated properly?
If you check all this and the problem remains with the suspect socket regardless of what lamp is installed where, I'd say the ballast is a good suspect. You may be able to purchase a ballast module separately through third party retailers or through the manufacturer. If you are electrically savvy, you can also often replace the ballast component itself with an off-the-shelf compatible ballast.
