I had a problem with my IQ-X recently, which I mentioned in the Igaro thread...
On the ride out to the start of a 300, the light was working fine, but something must have happened during the ride because when evening came round again, the light was refusing to work at full power - it seemed to be stuck in daytime mode. I didn't get a chance to do anything about it before the 400 the following weekend and riding those dark country lanes, it felt like I was back in the 80s with the amount of light I was getting.
Subsequently, I did some testing - removed the rear light and Igaro, and the IQ-X worked fine. Tried it with the rear light and no Igaro and it still worked fine. Tried it again with the Igaro and... it still worked fine.
I'm guessing that perhaps a wire had come loose and simply by reinstalling it, I had made the connection stronger. I don't understand why this would mean partial lighting power rather than no light at all though.