Yes, halogen lamps do become more dim and more red over time, and this will increase your exposure time. Compared to the more standard types of incandescent lamps though, halogen lamps tend to be pretty stable for most of their lives, then die suddenly as you've noticed. Perhaps this lamp is an exception to the rule. In any case, the lamp is very likely nearing the end of its lifespan and should be replaced. Examine the socket for traces of corrosion or oxidation. Clean or replace as necessary. That should fix it. You might also want to check the inside of your light mixing box if the enlarger is so equipped. The inside of the mixing chamber for my Chromega II lamp house had yellowed over time from the heat and intense light of the halogen lamp. I re-lined it with new white styrofoam and the difference was dramatic. Printing speeds were cut in half and contrast went up I estimate about a grade with papers I'm using. I had to recalibrate everything, but now I know. When the foam liner in the mixing box starts to get old, I change it out.