If it's acting like the description below, the trouble may be in the AFM.  Try a google lookup on vanagon syndrome.

  Copied from first entry:

>> You are on a longer trip, the wasserboxer has been humming along for several hours and runs really good. Then all of a sudden, out-of-the-blue, the engine cuts out.  The vanagon lurches, loses power, and starts bucking.  You pull over and shut it off.  A little later, you restart the motor, and it runs just fine.  Sometimes this might not happen again for a long time, or if you are not so lucky, the vanagon might start this syndrome all over again after only a few minutes of driving.  Sometimes the cutting out is very slight, barely noticeable...<<

George/ATL