Try this test, Jerry...

Get a digital multimeter.
Disconnect your black box from your rigger cables, to eliminate this variable.
Go out, away from the marina and other boats, and lower your downriggers to where you normally fish (say 60ft).
Set your multimeter to DC, then touch the black probe from your meter to the negative post on your battery, and touch the red probe to your downrigger wire... measure the voltage... this is your boat's "natural" voltage. Hopefully it's in the range of 0.5v+ to 1.0v+. (If it isn't, you have other problems, and either need to add zincs to your boat or you need to trace down an electrical leak on your boat, like a broken wire or a bildge pump or something leaking voltage.)
Now keep holding the probes there (one to the battery and one to the rigger cable), and have someone push the button to bring the downrigger up.
My guess is that your voltage will skyrocket, to 7.0v+ or so...

I've never done this test myself, but I've heard that the Cannons induce a high positive voltage on the line while retrieving the downrigger ball... these riggers have an internal circuit that is similar to a ground fault circuit, when the ball breaks the surface of the water, the downrigger senses that, and stops retrieving.
I'm guessing what is happening is that your black box is sensing this change in voltage and is trying to override the high voltage and regulate it back down to the "induced voltage" setting that you've set it to.
Obviously, if your downrgger cable voltage ever goes above 1.0v+, you're turning away fish... 7.0v+ is really hot, really bad...

If this high voltage isn't happening when you're retireving your downrigger balls, leave your downrigger ball in the water, keep holding the probes where they are, (black to battery neg post and red to rigger cable) and then have someone turn your electronics on and off, key the VHF mic, etc... look for any big changes in voltage... you might have some other electrical problem on your boat, like a weak or corroded ground wire somewhere...

But, I've heard of this Cannon problem before, and I've talked to guys that have had both electric Cannon's and electric Scotty's on the same boat, and the Scotty's caught way more fish, and it's my theory is that this high voltage on the Cannons is the reason. The Scotty electrics use a mechanical method of sensing when they should stop retrieving... you put a plastic stop on your downrigger line, and when it comes up, the rigger senses the stop and stops there...

Long story short, it's time to trade those Cannons in for a pair of Scotty's... (or just get one Scotty for now and see if it catches more fish than the Cannon)

-N.