Doug,
At the very least I would check the circuit that connects the smoke detectors together and the circuit that it is powered from.
It sounds to me that the detectors aren't too happy with the supply voltage they are getting and are doing a slow burn.
To turn off the circuit, is just stupidity, you might as well not have them at all.
Maybe it could be a manufacturing fault with the devices, it might pay to contact the manufacturer to find out if any others of this particular model have suffered the same fate.
I would tend to hope that you would get more than 2 years out of a smoke detector.
