Vapor Lock question
You're not getting vapor lock, you're getting more of a vacuum lock.
It's got to effect performance.
Does the inside of your cap look like the picture that 67Dart273 sent ?
Take your cap and line the inside with a paper tissue, then put your mouth on that tit and suck on it.
Do you get any air ?
My 2 cents.
That's a nice looking shifter. What Brand is it ?