Laurel Bottled Gas refused to refill a tank for me last week, saying they've decided to switch to a swap system.
That's too bad since they were in the neighborhood of Maryland Homebrew.
I've been driving around for a couple of weeks with my empty spare 5 lb tank in the car, hoping to catch the HK guy at home. I go through his neighborhood a time or two a week. Went by 3 times - nobody home. The phone number is now disconnected. Will go see the pirates at MD Fire Supply now.
Brewed a double batch of Northern Brewer's "Oak Barrel Porter" this weekend and pitched Wyeast English Ale that I built up into two starters. Glad I used blowoff tubes. It's the most energetic ferment I've ever seen. Here's a video.
http://vimeo.com/user5623750/review/53854948/deb8b52af7
I have that happen on my DIY plate when I use a short stir bar. it doesn't connect with the plate magnets as well as the longer ones I've got. also my longer bars have a little ring or ridge around them. that may also be a factor.
You might experiment to see if a longer/shorter bar would work...
If you can remove the cast iron, and only the cast iron from the burner setup, try putting it in your oven and setting it on clean for two or three hours.
I do that to burn crud off cast iron cookware that needs to be reseasoned. Turns everything but the iron to ashes.