I had exactly the same problem that you described.  Last spring I did a 2000
mile trip and on 3 separate occasions RPM suddenly dropped, forcing me to
pull over.  I had an extra control unit with me, and so assuming that this
was the problem the first time it happened, I exchanged it.  This seemed to
eliminate the problem, but about 3 days later I had a reccurence of power
loss.  

After returning home I spoke with a VW serviceman who had seen the problem
before.  He said that when driving for extended periods of time the control
unit on occasion gets confused by an overload of input from the air sensor.
Apparently VW came up with a patch to the problem which is a small 6"
harness that attaches to the sensor.  Before my next long trip I plan on
contacting VW for details on obtaining the harness.

Has anyone else experienced this problem?