HEy
I have recently purchased a 94 gts4-t auto.... Firstly, 350 to 400 kms is an average mileage that i get with a 95 prem.... Secondly, i had a similar with the lack of pulling (reving passed 4 to 5 grand) and considering it was my first skyline i had no idea.... soo i sent it to a mechanic! My car would suck the gas and not even go....!
Thing that were changed! (similar to what cubes sugested)
1. changed spark plugs - NKG
2. air flow meter - off a 300zx will work
3. ECU - wreckers? possible
The cold start seems to occur with alot of turbo cars especially gts4-t skylines....
The mechanic change over all over the above however the spark plugs permantly fitted the problem... appparently i was running some "performance spark plug" with incorrect gapping.... Try that the spark plugs could expalin why u are using alot of fuel and hitting that 4 to 7 grand flat spot! once my were changed over...... it snaps the 4 to 7 grand mark! and it shouldn't cost u an arm and a leg to place them! fu*K u need to change them eventually anyways!
The ECU was replaced because the car went spaztic... and i mean spaztic.... won't accelerate at all.... like my 1.5 litre paseo had a quicker take off! and then it would kick in and pull forward then drop off.!
Hope that helps!