Just not going fast enough Yoda!! Mine only went down when I had the alloy caps on at around 140+. Think the centrifugal force gets too much, and with the extra weight of the alloy caps, it bends the valves allowing air to escape. Funny though, never had that problem with the car, and that regularly travels way in excess of 140. Must be different valve stems or something on the car.....
Errr as Mrs Yoda reads these posts too , yeah I aint going fast enough mate. Let slip on holiday to mate what beastie will really do and well she aint said anything, but I reckon I'm in dog house.
Never seen any official "bad press" about them, just my personal experience and a few of the others on here have experienced it. Think under normal road use (as in within the speed limits) they're ok, it's just at high speeds they are a problem. Might be able to overcome it with different valve stems or something. I only ever had a problem on the front wheel at 140mph+ , never a problem with the rear. Never had a problem with the alloy caps on the car either which also visits some very naughty speeds By the sounds of things, it's not really a "they will cause this", more of a case of they might, but personally wouldn't take the risk myself