Extremely wierd hesitation at low rpm

K guys this condition originally started about 1 and a hlaf yrs ago with extremely long cranking times in slight cold (all summer it runs fine). Yesterday it cranked for about 15 minutes luckily i have a red top battery. Its got 80,000kms on a rebuild 350 and just lately it got really bad to where i got this wierd hesitation. If u put ur foot to the floor the engine will stumble between 500-1500 rpm but will not go higher, rev slowly to over 1500 and the thing runs great after. It does not stall altho it feels like it should. I just replaced the fuel pump/filter and it helped run and start better in general but still bad hesitation. Any thoughts on engine issues? smokes slightly on start up which it didnt b4. should i pull the plugs?

First, you don't mention the truck's year and total mileage. That can help.

I'd do all the regular tune-up checks...plugs, wires, coil, cap, rotor. Also check the PCV, EGR, and IAC valves for good op. Check the timing. Check for play in the distributor rotation. If equipped, check the O2 sensors. Finally, check for vacuum leaks around the intake, TB, and vacuum lines. Most of the time it's one of these things. Post back with your results either way. Good luck!