I've got the same sort of issues with my udp, and wet weather, if its wet out, and i use the rear defrost, the alternator belt starts to squeal, once i come to a stop, you can hear it start to squeak just a little, and then as soon as i go to take off from a set of lights, the belt starts to slip really bad. If i pedal it, it will get more violent and then start to grip, and shut up... sometimes when the sound pisses me off i do that...

However sometimes, its the ACC belt that slips, i have this occur a lot less often. The only time the power steering and ac belt slip, is when i turn on the window defroster, sounds like it makes the belt slip once the ac kicks on.. not sure why my ac comes on when im defrosting.. but yea, again the squealing only starts when i take off, not steady cruising, and only if the A/C or Rear Defrost, is on.
Anyways, so i'm thinking i'm gonna grab some new belts. the ones im using are in inches.. and i remeber thinking if i could get a half inch shorter, i would, but Canadian tire didnt have it.. so maybe a different brand, that measures in mm. or cm? i've heard the gates belts fit much better, then the power torque belts i have now.
New Sig Time.