Quote Originally Posted by gainer
At the beginning of each session and at the end, remove the enlarger lens and measure the incident light at the lensboard.
If you measure at the lensboard, make sure the bellows are always at the same extension, otherwise the measurements will be meaningless.

I'd still look into using distilled water for the dektol, and seeing if the inconsitencie remains.

Another thing no one else mentioned is paper batches. Although I'm sure different batches wouldn't result in 25% difference in time, exposure to heat and/or cold might. You mentioned that the darkroom is in your garage, do you store your paper there?

Same goes for the dektol, I'm sure.