Les, if it was the enlarger or timer wouldn't it show up within a session? Ever? Especially in an 8 hour one. When I try to duplicate a print that I am happy with in another session there is often as I said an approx 25% difference. But when I make a few changes to bring the print to where I want it to be, from that point on within that session if I make duplicates they are consistent.
There are variations in temperature in different sessions usually less than +- 5 degrees from 68 if that could make the difference I don't know. But if the temperature is cooler I'll keep it in the soup for 3 minutes or warmer than 68 in the soup about 2 1/2 minutes. I develop face down as to eliminate additional chance of fogging and don't even look at the print until I take it out.