What changes at about 3000 rpm?
What sensors does the ecu stop looking at or whatever at about 3k ?

My 300zx runs like two totally different cars above and below 3 grand. Above 3k it runs like a champ. Pulls hard. Runs smooth. Etc etc. Below 3 grand, it breaks up. Makes no power. Just runs all around poorly. But. The idle is generally smooth. You can hear a very light miss in the exhaust at idle.
The engine in this car previously did the exact same thing.
Any ideas?