Air Pump....

Well, I just went to LBS and took them my bike since I am jumping gears and found out the tech that installed the cassette on my new rear did it wrong, but it's fixed now. 

I also brought the pump in and had the same issue with it jumping off the valve stem...they gave me a new better pump in replacement of the other pump...yay.  Thumbs up for this LBS in fixing the issues, no questions asked!

you knocked out two birds with one stone there!

I had the same problem at first (pump not getting a good seal), but after a few times, it didn't happen anymore.

almost took it back.

I air up weekly and ride about 5 times a week.  The pump hasn't worked since it was given to me 3 months ago.  I had enough of it last night after bending 1 valve, blowing 1 tube and then not being able to air up the last tire. Grrrrr.  I am happy now.  The head on this pump is a "smart head"  My other one had 2 separate holes depending on if it was a regular mtn bike tire or the road bike tire.  This one fits both.  Fingers crossed this one works like a charm.

I know my pump has a screw cap kind of thing on the end that holds the rubber seal in place.  If that screw cap is loose or not tightened enough, then it will not hold the rubber seal tight enough to get a good seal on the valve.

I know your problem was solved with a new pump, but I figured I'd add some value to the thread in case someone else has a similar issue.