Dang Richard, I must have been dreaming about your problem last night. Woke up with images of veggie snot dancing in my head
Somewhere during REM it came to me that you have that electric heater inline as well. I wonder if the element is causing some of the crud you are seeing? Maybe try running without that for a while. Coolant heat should be plenty.
Also keep a sample of the stuff. Experiment with it. See what it's melting point is. Sputter test it for water.
I've heard Methanol is a great cleaner of veggie crud, but it's also great for causing nerve damage and blindness. Use full precautions and in a ventilated area if trying that.
Got any pics?