So it's really only speculation at this point that it's ignition-related ?
( the tach indications point strongly in that direction I realize. )

just to jog some diagnostic/testing thoughts......
it would be interesting to rig a timing light so you can observe it while
driving.
If it goes dead when the engine starts shutting down.........that would be a
clear indication it's the ignition system needs focusing on.

you don't have a spare distributor do you ?
Sure would be  nice to pop in your back-up spare  and see if that changes
things.

traditionally, when electronic ignition fails only when hot.........you
think of the igniter.....
and this being a 2.1 ...........and that function is in the ecu, though I
almost never see a bad ECU.
Scott
www.turbovans.com

----- Original Message -----
From: "Greg Potts" <greg@POTTSFAMILY.CA>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Saturday, February 28, 2009 10:28 AM
Subject: Re: Engine stutters, tach nosedives. Then, just as suddenly,
it's,normal again.

> Hi Keith,
>
> The failure we are discussing has always happened on the highway, with a
> fully warmed engine. Every time.  FWIW, only about half of my commuting
> is highway.
>
> I had a coil fail in my baywindow camper on vacation back in 2006... It
> felt exactly like the problem I am seeing now. In that case it was a bit
> easier to diagnose since the ECU does not control the spark in an
> L-Jetronic engine.
>
> Another possibility that came up in P-mail discussion is the ignition
> switch; but I did not notice any of the switch-dependent accessories
> stutter during these incidents.
>
> Happy Trails,
>
> Greg Potts
> 1973/74/79 Westfakia "Bob the Tomato"
> 1987 Wolfsburg Weekender Hardtop
> www.busesofthecorn.com
> www.pottsfamily.ca
>
> Keith Hughes wrote:
> SNIP
>
>> Shorts in the coil
>> *usually* start causing problems when Hot, but not always.  Since,
>> however, the coil heats and expands as you run the engine (flexing the
>> primary/secondary coils in the dielectric oil), you should notice a
>> difference in performance (either better or worse) the longer the engine
>> runs.