1987 5ktq "Tantrum" behaving badly as e-test is due

[Steve Sears]

Hi all,
I've been troubleshooting some problems I've been having with my 5ktq as 
e-test time approaches.  Idling between 1400 and 1600 rpm on about a 1 Hz 
frequency - started that way one morning late last week and has done so ever 
since.
I've chased the "usual suspects": ISV is new, clean and in working order, 
replaced the plugs, cap rotor, dizzy recently due to a weak hall sensor. 
Throttle body switch is in working order after a short stint of 3k idle that 
started 2 days ago when troubleshooting (wire broken in connector).  Fuel 
injectors (leaky and possibly wrong type???, replaced with non-leaky correct 
used ones, cleaned and bottle-tested), cold start valve clean, O2 sensor (2 
years old generic) replaced with (used, spare) Bosch with no effect.
I know there's a leak in the wg diaphragm that I haven't addressed but I've 
been off the boost and it was noticed a while back.
I made up a pressure tester from stuff from the local Home Despot and 
applied regulated pressure to the intake at the Michelin Man hose - some 
small leaks at 1.8 bar (dash) and a massive leak around the butterfly shaft 
on the throttle body.  The parts engine I have has a TB that is slightly 
different than the one originally on my car - no dashpot, slightly different 
idle screw location, and a small hole through the lower section of butterfly 
where the original didn't have any.  I installed the new TB, it held way 
more pressure than the original (only a slight shaft leak at 1.8 bar), but 
the car still idles between 1400 and 1600 and now it stumbles when starting 
and running.  A couple of questions:
1.  Are the two throttle bodies interchangeable?
2.  What am I missing that would cause such an oscillating idle?
Many Thanks,
Cheers!
Steve Sears
1987 Audi 5kTQ
1980 Audi 5k
1962 and '64 Auto Union DKW Junior deLuxes