Since Borax and Metaborate, at least when they are dissolved in water, are very similar in composition (same ions, just different ratio), yet have very different solubility limits, there is a good chance that the solubility of Borax strongly depends on pH of solution. Whatever else is in the D-76 formula might make the difference between 47 g/l and 60 g/l possible.

There seem to be two factors at play here: first, you seem to store your stuff at very low temperatures, that's likely the reason the Borax recrystallizes. Second, dissolving Borax close to its solubility limit is slooooow, which means you needed to heat the dev way beyond room temperature to dissolve the precipitate.

There seem to be two options available: Mark Overton aka albada posted in his D316 thread that Metaborate/Borax will dissolve in Propyleneglycol, which means you can make a concentrate that you add right before you mix your working solution. If that's too complicated, and it will require some research on your part, and lots of PG, the only remaining option seems to be storing your dev at or above 20C/68F.