More likely is at increased rpm levels the amount of air being compressed onto the head of the valve helped it to seat a bit better. Back in the day we called that phenomena pumping compression.
the hydraulic pressure in a lifter forces the internal piston in the lifter to extend, increasing its length, higher pressures would not make a valve close any differently as the lifter LIFTS the valve open, and unless you have a Ducati Desmo, it is all up to the valve spring to close the valve.
In all seriousness and not wanting to piss you off any more than I have let me say this.
You seem to be very determined and don't give up easy, which I find to both be great attributes.
You seem to enjoy getting your fingers dirty which says to me you aren't one of the pompous schmucks of the world, just a down to earth ordinary guy.
What I also notice about you is your lack of understanding of an internal combustion engine.
How about going to the library or downloading onto a NOOK or whatever a book or two on the basics of internal combustion engines.
Side question for anyone: does a valve spring pull the valve shut?
I bet you would enjoy reading Kevin Cameron articles, especially his basics.
He gets so far over my head so fast sometimes I get dizzy trying to comprehend him, but for the most part he explains things nice and easy. And normally with a good story added in.
I am glad to see you find a major problem and hope it cures the van and you don't have to spend any more $$ on it.
Enjoy 2011. The grumpy old bastige know as Ozzy.
