ur-quattro misfire?

[quk@sievers.com (Phil Payne)]

OK - it's brains trust time again.  1988 ur-quattro, MB engine, two knock 
sensors, MAC12.
 
It's damn cold here (by local standards) so I'm not playing with the car 
tonight, but I'm going to have to, very soon.
 
The problem _seems_ to be a misfire under load.  Idle is smooth - level and 
constant speed shows a few symptoms, medium load shows a _h@ll_ of a lot, high 
load seems to smooth it out again.  It's _very_ intrusive - noticeable to 
passengers.  Up a 1 in 20 hill in fourth gear with the accelerator 1/2 down is 
_very_ unpleasant.
 
I haven't pulled the plugs or checked the wiring yet - the ignition harness is 
relatively new.  The rotor arm actually burnt through at one point last year, 
so I've got a few obvious things to check.  I even thought about water in fuel, 
but it's had two fresh tankfulls since the problems started.

Any more theories?

--
 Phil Payne
 phil@sievers.com
 Committee Member, UK Audi [ur-]quattro Owners Club