Got a 99 civic DX with the D16Y7, its been doin this for about 4 months. When at highway speeds if im barely touching gas pedal until about half throttle it bucks/hesitates very noticeably, and idle fluctuates when at a light/stop sign. When pushing it past half throttle the bucking smooths out and only really does this when im on a highway. ive cleaned the IACV before, new fuel filter, injectors, plugs, air filter, plug wires, and distributor cap all within the last year for other issues i was having. It sounds like there might be a vaccuum leak around air box but i cant find it using Carb cleaner, and i have no CEL its got 150k on it i put a timing belt on it around 130k. anybody got any tips on what to look for? This is first vehicle ive ever had that wasnt a diesel.

 

I too have experienced the same issue as you described. Read into numerous threads for quite awhile.. All of which never helped my situation.
The one thing that resolved the issue that had been ongoing for some time was after replacing the fuel pump, since it was 17yrs old. The sock on it was black in color on the inside and partially clogged I take it.
Just maybe you have the same problem or some problem in the fuel system? Just a thought..

 

ive replaced pretty much the entire fuel system except for the pump, i guess ill look into that thanks