Please Help!

I just had my car at the dealership and had the coolant temp sensor, idle air stabilizer replaced, and a general tune-up. But 2 weeks later(today) my car is hesitating at low throttle. If you push the throttle a bit it kicks in and runs fine. but as soon as you try to maintain any speed it starts hesitating again. The idle drops really low for a second (but will idle fine) when you take your foot off the gas. I looked in my Bentley manual, but of course there's like 10 different things to trouble shoot, i just don't have the time or the facilities right now to check them. Has anyone run into this problem, and if so what was it? I am probably going to go back to the dealership with this problem( but they are out of town, so it is a pain for me to get there) Any Help would be great.