Odd. In the past week (as it has actually gotten cold finally in New England) we noticed this same thing happening. Aquasol HV...I figured it was our bulb. We do 100-200 screens a day, sometimes excessive double post exposure, high density screens coated 300µ-600µ thick with 300 to 600 second exposures. It's been 3 months, figured it was time.
Well, put the new bulb in today and barely dropped our exposure times. I wonder if we might be afflicted by the same thing...never noticed it in the cold before. But perhaps it's the batch of emulsion?