Car runs rough/stalls at started. then hesitates...

[Fastcat]

I just bought an 89 T-bird (3.8L engine) and the guy I bought it from told me it has been sitting for about a year and a half because of a blown headgasket. He replaced the headgaskets, got the heads checked for cracks and warpage, and had a valve job done on them recently (about 2months ago). He also replaced the T-stat and waterpump. If the car has been sitting for a while and you start it up it will die unless pump the gas pedal. Then when it stays started it idles really rough and if you pump the gas the engine will hesitate very badly... like it isnt getting air or fuel. or if you put it in gear it will shake badly then stall. But if I let the car warm up for 5min or so the idle will smooth out but the hesitation will still be there when i hit the gas until the car warms up more. When the engine reaches the normal operating temp it runs 95% better than it would when I 1st start it up. But if Im at a stop light or something and go to take it it will hesitate for a couple of secs then take off. it really doesnt hesitate while driving, just on take off and an occasional surge wile driving. Could this just be bad gas? Ive put 2 bottles of watter remover in the tank, octane boost, and a gallon of preium gas. I also replaced the IAB and I checked all the spark plugs and they are in good condition.... the firing order is correct also. What do you guys think?