Engine Performance problem
1989 Nissan Truck 4 cyl Four Wheel Drive Manual 299896 miles

I have a 1989 Nissan D21 Pick up with the Z24 DOHC i just got it and i have put alot of money into trying to fix it. What it does is when i crank the truck up i have to give it constant gas to get warm and as i do so if i give it about 1/2 throttle it wants to completely shut off i have replaced the throttle position sensor and it still has the problem and as i shift once i get up to 25mph in 1st gear it does it and in 2nd gear about 45mph it does the same thing and and so on and so on through the other gears what should i look at