2002 Mercury Cougar Hesitation/Surging/Hitching

My check engine light has been going on and off for a while now, and then my car started to hitch. I had a diagnostic done on my car and ended up having the EGR valve replaced. It didn't help at all. Basically I'm noticing that the "hitching" occurs only when I'm holding the gas at a consitent pressure. Holding a steady speed and not doing an on/off the gas pedal deal. It feels like my engine just wants to give up, like it's not quite getting gas, and it's going to stall out. I hear no noises from the engine when the hitching occurs, and my RPM needle isn't effected either. Thank you for your time!