My guess would be you've set your camera to auto white balance and it's balanced it's self to the tungsten modelling lights..........
This makes sense to me. But it would make more sense if Russ' problem were consistent and not intermittent. That doesn't mean, however, that this is not the answer. I believe the problem will be either of this nature or in a faulty flash tube as Les has suggested. Or, it may be a combination of the too given Russ' flash heads and tubes are 10 years old.
Peace! Sidney
