Are you certain that both the transformer and the dimmer are really matched and intended for that purpose? Dimmer outputs at lower lighting levels are very different from typical AC power.

So by proper dimmer, you mean one suitable for transformers? Check that the transformers are suitable, but, more importantly, check you aren't overloading the transformer with too many lights!

We had this problem with our low voltage light fixture.
Turned out it was caused by a short in the wiring after the transformer.

First timer is on the right track. I have also run into this where larger lamps than recommended whre installed. If you have a transformer rated at 100 watts and you install 3 50 watt MR16's the tranformer will burn out, maybe not right away but it will burn out.