Help car is acting up

[mppaintball]

So i just finished replacing the heater core. Take it for a test drive and one of the hoses blows off. I clamp it down better replace the coolant and go for a drive again. The heater is working perfectly but the car in hesitating when i give it some gas in 3rd to 5th gear. When i gas it it is kind of hesitating and bumpy. I tried to get any bubbles out of the lines but all i did was open the radiator cap basicly. Anyone know why it is doing this? Could it be the fuel filter? But why would it only start doing this after i replaced the heating core? help please

Ive been told it might be a vacume leak so i tryed to check the only way i know. By spraying some carb cleaner on the vacume lines under the hood and seeing if it changes the idle of the car. So i tried that on the ones i could see and it didnt do anything.

Also could this may be it. Im do not remember unplugging it and im not sure what it plugs into

 

[atrainsgt]

Try resetting the computer. Hesitation could be several things.

 

[mppaintball]

it is like the engine is fluttering, but runs like a dream when it is in neutral

 

[atrainsgt]

it is like the engine is fluttering, but runs like a dream when it is in neutral

I had a similar problem and it was just the o2 sensors. Mine was fine in neutral as well. Only ran bad under load, and didn't always run bad. Just when the conditions were right did it sputter.