[Gmecm] Strange effect while trying to tune my GM-EFIed beast

[Tomas J. Sokorai Sch.]

I'm in the tuning stage of the conversion from a mechanical FI '82 I6 BMW  to 
a '727 ECM with $8D code and DIS ignition, and last set of modifications gets 
me the following strange effect:

The engine runs cold very well, a little rough, but pulls strongly and at any 
RPM.
But when it gets near its working temperature, it starts to have a strange 
cut-off at exactly 2000 RPM. It feels exactly like a hard revlimiter.
The harder I put the foot down, the more pronounced is the cut/resume effect.

See http://www.xperts.cl/temp/revlim_problem.png
For a screenshot of a datalog I'm slowly trying to get the 2000 RPM and it 
starts jerking. Then I try to put a bit more throttle, and I get only harder 
cut/resume cycles.

At the strong oscillation, I get some loud "bang"s on the exhaust too.

Any ideas of what is happening?

-- 
Tomas J. Sokorai Sch.