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Kinda hard to diagnose over the phone as there can be a multitude of things that can cause this but here are some checks and info. With KOEO disconnect fuel pump safety connector on the back of the air flow meter and make sure the frequency valve is running (audible test). Passing that start car and do a duty cycle % test at the test plug on the relay plate, looks like a round relay plug connector. I don't have the pin diagram or the spec off the top of my head but should be easy enough to find. I want to say at idle the duty cycle should be in the 55% range.
Have you tried disconnecting the O2 sensor, if so what happens. Have you looked at the timing, does not sound like thats the issue but you need to start with basics of timing and fuel mixture.
On the 86-89 USA 911 turbos the lambda system runs closed loop up to 3000 rpm or a 65 degree throttle angle, once you pass either of those threshholds it goes in to open loop so if the engine runs ok past 3000 rpm it sounds like something is leaning it out during closed loop (O2 sensor ?)
Again kinda hard to diagnose without being there or lots more info. Hope this little bit helps.
Richard
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