I've been using this developer for the last two years and have not experienced any sort of problem as you describe.

I mix it at the recommended 1:9 dilution but mix it in 1 quart batches. My darkroom sessions usually last 8 to 10 hours and I usually average 30 to 35 8 x 10 sheets through the solution.

Developing time for me is 1min 30 sec and that remains constant through out the entire session.

I don't get to spend a lot of time in the darkroom so I only purchase the developer in quarts. The concentrate is stored in a glass bottle and closed with a Vac-u- Vin stopper to remove the air as the bottle empties. A quart will last me several months and there has been no problems with the concentrate even when I reach the bottom of the bottle.