Does it do the same thing if you manually downshift into second? If it does, then it's a misfire.  Those are usually caused by weak spark (bad plug or wire) or a lean condition from a vacuum leak or bad fuel pump. A timing light will fire with a bad wire.  Another easy test is to look at the running engine in the dark and see if you can spot any blue sparking from current leaking from the sparkplug wires or cap.

Stuart 

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of John Rodgers
Sent: Wednesday, September 24, 2014 5:41 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Engine Shudders -2.1L WBX.

I have a 1991 Carat with auto tranny. On floorboarding to go up a hill or
kick down into a lower gear to pass, the  engine seems to drop a cylinder
an shudder as if not firing. I have a Shoebox Electronic ECU Monitor hooked
up and have the engine fine tuned and all the numbers are good and the
lights are correct, yet I get this shudder anyway. I checked with an
induction timing light the firing of  each ignition wire up 2500 rpm. Each
was good. Yet it still shudders on pressing the pedal to the metal.

Any ideas?

John the Gimp