Sounds like the voltage stabiliser, but check the contacts in the lamp holder. I have had issues with my De Vere where the contacts get a film of oxidation. It's an old enlarger I bought mine in 1975, so check the holder it's like having a resistor inline when the contacts aren't clean.

Is it the Durst stabiliser, these are designed to give a fast response but I have other 12v power sources for similar bulbs and they are slower.

Ian