Need held identifying hesitation source on 87 celica

I have an 87 Celica ST with a 5 speed. Im having some slight hesitation at low RPM's; however, the hesitation goes away after the car has been driven for a while. Idle is great (650-750 at all times), and so is the acceleration. I already regapped and cleaned plugs, changed fluids, checked timing and timing belt postion,and ran the diagnostic, but nothing has helped. No injector leaks either as i changed all of the gaskets when i dropped in the new engine. Im thinking the problem has to do with the fuel system, maybe the fuel pressure regulator or pump. I also have a difficult time with cold starts, although i have replaced the cold start injector.(Just thought i would throw that in there if it is any relevance.)

Do any of you have any idea what the problem could be? Thanks for the input.

do a vaccum pressure test, and clean out ur distributor and check your rotar, those are great places to start...check all vaccum lines, one vacum line loose could mess up alot of things