Nick, had the same issue using a Beseler drum and roller motor. The fix I found was to increase the total volume in the tank. In my case, I was using 300 ml for 8x10 - changed it to a full liter (1000 ml) and the issue seemed to go away. The problem can occur when the developer begins to create a wave pattern (for lack of a better description) which seems to be disrupted by taking the tank of and moving side to side or in my case by altering the amount of liquid in the tank. Can't promise it will work, but considering the low cost of mixing Pyrocat it is worth a try.

Good luck, let me know if this helps.