It's hard to say exactly without seeing a picture of the problem, but I had something similar happen to me (and always on the best prints!). I knew it was contamination but couldn't figure out from what. Since I use my fingers to move and snatch the prints (with nitrile gloves on of course) I realized the contamination was coming from that -- so after every print I tossed the gloves, and put on a new pair, and my problems disappeared. I don't know what your process is, but that might be something to consider (same for using tongs).
And, like the others, I always develop at a much higher temperature (around 30 Celcius to start). It keeps developing times shorter. Keeping the developer tray in a larger tray with a warm water bath helps keep the temperature consistent longer as well.
Anyway, let us know if you fixed the problem.