You probably are having water evaporate from the developer and thus concentrating it and forming a super-saturated solution. You might try adding some distilled water after your printing session to bring the concentration down.

What I have done is pour the Potassium oxalate into my warming-container, leaving the crystals behind. Then add the distilled water to the crystals, warm it up and when they are dissolved, pour it in with the rest of the developer.