Any other Bi-fuel (CNG) owners out there?

[sdhamblin]

I just picked up a bi-fuel F150 this past weekend, and I'm having a bit of an issue. Folks I talked to before I bought it said it wouldn't start on CNG if it had less than 1/8 of a tank, but I was having a rough time getting it to use CNG this morning when it had over 3/8 of a tank. Then I filled it up, and it still kept using gasoline instead of CNG. Finally after restarting a dozen times (stop lights etc), and driving 8-10 miles, it started on CNG and stayed on CNG, but for a while the fuel guage was wandering around, up and down, from 1/4 to almost full. Any ideas whether these are related, what the problem is? Thanks!

I tried searching the forum for CNG and found nothing-is there really no one that's posted a CNG/Bi-fuel issue on the forum, or am I just missing them?