I have a holding tank system that goes head to holding tank without a direct discharge line. Recently I've noted that every now and then I will get a little bit of "back pressure" on the flush pump (Wilcox Crittenden head mate) and a little geyser of "stuff" shooting out the vent outside. This is accompanied by a very unpleasant odor in the cockpit (but not in the cabin) that lasts 3-4 minutes. Has anyone had this problem before and what needs to be done to fix it? I keep a "tinkle only" policy and there really isn't anything except liquid going into the system (occasional TP when the ladies go). Any insight would be greatly apreciated as this can ruin an otherwise lovely sail in a hurry.