Just bought a 1990 GL, PO had brand new heads installed recently(hint).  When the van gets up to operating temps, it loses power under load, esp. when going up hills.  A restart would restore the power for a short time.  Another clue was that the plugs were black and fouled from running rich.

Since this is not my first VW, I decided NOT to start the expensive, 'replace everything until it goes away'.  I cleaned all of the ECU grounds (the big braid ground from the head was just hanging there!) and it ran better while warming up, but it still lost power under load when fully warm.

Searched the archives, and found a few articles about a 'hidden' ground wire on the driver's head under the intakes - Ground Point 18 or GP18.  This ground provides path for the O2 sensor.  I look, and sure enough, its just hanging in mid air - probably from when the heads were done.  Had to remove the intakes to clean and rebolt that sucker down.

Now the van drives flawlessly and has plenty of power up hills.  Anyone who has had heads done by a shop and is experiencing weirdness might want to check GP18.  I guess the O2 sensor was sensing a lean condition and telling the brain to enrichen the mixture?

John Haddad