ML 320 Throttle complete cuts out (sporadically)

My Mercedes ML 320 runs fine sometimes for days and then after I left off the
gas or stop at a light, then try to accelerate, the throttle is completely gone.  
Car still running.  No check engine light.  Turn the car off and on, good to go.  
Sometimes for 20 minutes, sometimes for 2 days.  Gets WAAAAY worse in
high altitudes.  THANKS FOR ANY HELP!!!!!