Steve: the timer is good to 10ms (on a ~25s exposure in this case), i.e. half a mains cycle since it uses a triac. See timer link in my signature.
I note that later DeVere enlargers have stabilized supplies; mine is the 1978ish model (Mk.I) 504 that precedes such niceties. Certainly a regulated DC supply would solve the problem, the only drawback being $$$ for a 12A 24V supply.
I think my solution will be "make a quick test strip before blithely exposing that 16x20 I setup 18 hours ago". I only got into this situation by fogging (during development) the final sheet I made last night and giving up in disgust; came back to redo it tonight and had my little issue. I suspect the airconditioner.