I read your intro post and when you brought up the forklift topic it made me wonder if you had anyone in that business in your area that might be able to give you a few suggestions for your idle problem. If you don't find anyone let me know, there's a forklift company close to were I work.
Also feel free to stop at the Ranger/B series forum, your Courier fits into our group.
I used to run a Bobcat that used propane, so there's another source to possibly check into. I wonder if the manufacturer of the propane kits would have some troubleshooting pages on their website..if there is a website? It sounds like some sort of valve body sticking and releasing to make it surge. I'll also ask my brother tomorrow. He put LPG on a 460 and ran it for a couple years and didn't have any problems that I'm aware of. ( Except a little less HP than with regular gas.)
If it were a sticking valve body it wouldn't run smooth with the fuel controller off, would it? I think there is a calibration issue with the fuel controller because I can't sense the adjustments it makes when the engine speed is over around 1100 RPMS. Even so, Forklifts don't idle this high do they?
I'm roadtripping so I'll have to look into this stuff when I get back in a couple weeks.