Neal's suggestions are good ones, but if you've processed film from this batch and it's been okay, then that would answer the emulsion batch question.

Other issues could be--are you using longer exposures than you usually do for these shots, and possibly getting a reciprocity problem? Is there some daylight from an unshaded window getting into the shot? Is it possible that the water has changed (municipal water systems might up the chlorine in the warm weather, for instance)? Have you actually checked the pH of the solution? Have you tried just filtering (the lens, not the water) your way out of it?