i have a 1985 300zx..i juz bought it..well on the way home of driving it...in neutral it will only go to 4,000rpms...and wen it gets to high rpms it used to make a whining noise but doesnt now....wen in all gears it only goes to 3,000 rpms..has a lil power..but loses all past 3,000 rpms...also theres no exhaust gases coming out of the exhaust..even when i get on the gas...but theres a lil smoke coming from under the engine...could this be the cat converter being clogged up?

 

i have the same or very similar problem with my nissan r32 rb20det.. same as you explained. when i smell my exhaust it smells sour, but there isnt any smoke.. i changed the fuel pump but that didnt help, so now people are telling me to change my mass airflow sensor. try checking it if there is any differense with it disconnected or when connected.
im not a expert but ive had this problem some time, and just now have a heard that the air sensors are extremly sensitive to dirt and might get damaged easily if you use hks filter wich i have. please mail me if you find out anything..

 

mass air flow sensor boys. that or your cat is clogged up. i just fixed for the same issue

 

Map flow would rock the car
Cat converter, you could floor it but the car would still lose speed , the engine sounds healthy and you can rev it very high but it still looses speed / try it going up a long hill / highway , not to far for its a pain in the azz to keep up with traffic
also , Does it feel like the transmission holding you back ,ifso , as you reach 4000rpm press in the shift button , and if the car shifts and lets you go 100 mph its a trans. modual