I recently installed a series of low voltage lights on a set of dimmer switches. There are eighteen MR 16 lamped fixtures with each having their own transformer and they are switched with four dimmer switches. One switch operates 6 lights and 3 switches operates four lights each. They are all wired in series back to the dimmer switch. My problem is with the three sets of 4 light fixtures. Each set has one light that will go off for a few minutes then come back on. This is happening to the same light in each set of four. What could my problem be????