Low Voltage Lights
I would not expect low temperatures to have this affect, but this is just a gut feel. How old is the transformer? How confident are you that the load is only180watts?
If this were happening at my house, I would be removing a few lamps from my circuit (to temporarily reduce the load). If this solves the problem, then I would conclude a new transformer is in order.
If removing a few lamps does not help, I would then be looking at wiring as a potential culprit. My reaction would be to remove all the lamps and measure resistance between the two conductors (also removed from the transformer tap). I would expect to find near infinite resistance. If not, I would be looking for a short or failed wire or connector somewhere.