I just went out and checked my cap and YES, the handle "wiggles" slightly when the cap is tight. It seems that you are manually resetting the CEL but I wonder if the sensor that is throwing the code is bad or if it hasn't had time to properly reset itself (a certain amount of starts or engine run times, etc). How long does it take before the CEL comes on again? Have you tried letting it go for a week or so of normal driving before jumping in and resetting it? Also, check to make sure your cap strap isn't getting pinched under the cap - that may cause a problem as well.

RL