I had similar problems with my tank for a while. I finally asked a buddy at the field for help troubleshooting it. He took the tank out and was looking through it toward the sun to see if there was any crud in it and he noticed that the vent line had worked it's way up against the side of the tank. We bent the vent line back a little and I haven't had a problem since. I'm glad he was helping me because I would have never found that myself. Might be something to look for.