I had this problem with a set of Bontrager Hank tires on my Weinmann DH-39 rims. It didn't matter how much pressure I had (usually ran 65-80psi), the valve stem would creep over time. In one year, I had 6 or 7 cut valve stems between F&R. I tried 26x3.0 tubes and it made the problem worse.
There was a couple factors at play contributing to my problem. One, my bike, despite it's diminutive Huasheng 49CC engine, was using a 4G through a shift kit. 1st gear is 25.5:1, so it accelerates pretty quickly off the line! 0-20MPH in about 3-4 seconds. Two, the beads of the Hanks just didn't play nice with the Weinmann hoops. The hoops were clinchers, so were the tires, but the beads were too small for the downhill-style wheels. Three, Hanks are beautiful tires, and I often used tire-shine stuff on em. That stuff seeped down into the bead and made it slippery.
I use Kenda Kiniptions now, they have fat beads. No sidewalls to shine up either, the whole tire is tread bead-to-bead. The valve stems stay put now!