help with 91 240sx odd bucking/surging problem

[wicked91de]

i've searched, but can't find any resolutions to this kind of problem.

car is a 91 240sx SE, bone stock KA except for magnacore wires.

a week ago (right after i purchased the car) i had to put a fuel pump in it do to the old one crapping completely out. now. a week later it has started an odd new problem.

when you first crank it up, it runs and drives perfectly. then after 10 to 15 min. of driving it starts an odd off and on bucking/surging. mostly between 4K and 5.5K . but sometimes it will start the bucking at around 3 and smooth back out by 4500. and sometimes it will fall on it's face right off idle then surge to like and take off hard. spark plugs look new and clean. fuel pump, filter and strainer are a week old. also usually if you tun the car off for a couple min. then recrank it, the problem goes away for a while.also. timing seems to be correct as well. not sure what else to try. anyone had this issue before??

[wicked91de]

after further testing today it seems that when it starts bucking if i back off the throttle to maybe 3/4 throttle. it stops and revs on up untill i go WOT again.