I have some GU5.3 12v dichroic halogen luminaires [5 x 20W in a string suspended on a pair of catenary cables]. These are fitted in a utility room. I avoided low energy bulbs because of the lights being switched on and off regularly as we pop in to tend the machines. Yesterday I hung a row of wall cabinets, and spent a couple of hours in there fitting them and hanging the doors. The bulbs noticeably dimmed during this time. The 230v-12v transformer is a big wirewound unit, not electronic, and it got quite hot. Once it cooled off, normal output resumed. Anyone else noticed this problem?
Hang a voltmeter off of the transformer output terminals to see if the output voltage sags as the transformer heats up. If it does, check the input voltage to determine if it is sagging too.
The heat could be from overloading the transformer or a high resistance connection(s).
New thought: Is the transformer a 60 Hz transformer? Running a 60 Hz transformer on 50 Hz will cause it to over heat due to less inductive reactance. It is the same reason 60 Hz motors operated on 50 Hz run much hotter.
I would suspect that your transformer is marginally rated for the load you have on it. Think temerature coefficients. As the transformer temperature rises so does the winding resistance, hence reducing the output voltage. As the lamps consequently dim and the filaments cool their resistance will reduce, so increasing load. I think I can visualise a potential runaway effect here which could kill the transformer.
It's rated 105VA, 12V output, 230v 50hz, with 5 x 20W bulbs. Made in France, BTW. I have 3 of these units in various locations. No probs with the other two, tested today. I'll check out the wiring tomorrow.