"Hesitating" Car fixed last March - now the problem is back!

I'm female and know nothing about cars. I have a 1995 Saturn SL1 with around 156,200 miles on it. At the end of 2010 my car started hesitating while driving, like it was going to cut off. It was worse when I accelerated. I did some internet research on what it could be, and in March 2011, took a short list of possibilities to the shop and told the mechanic to fix all of it. Overkill? Maybe, but I didn't have time for trial and error. Here's what they did:

Fuel System Cleaning: Chemically clean intake system and combustion chamber
Chemically clean fuel lines, fuel rail, and injectors w/ an additive added to the fuel tank.
Chemically clean throttle body, throttle plate and air intake bore
Replaced spark plugs, spark plug wire set and all secondary ignition wires
Replace fuel filter
Synthetic oil change

Problem solved! They advised me to never use Arco gas, which I always have. Since then I use either Chevron, 76 or Shell.

But now the same problem is back.

It seems to be worse when it's cold outside, but it still hesitates even if the weather and/or the car is warmed up. I recently took it to the same mechanic who said they can't find anything wrong with it. And, no, I didn't ask him what they checked. They changed the oil and oil filter, and I needed to have the valve cover gasket replaced, which they said probably wouldn't solve my problem. They were right. A few days later after my tank was almost empty, I put some fuel injector cleaner in it, and it didn't do anything. I have no problem starting the car. Putting the car in neutral every time I stop keeps it from hesitating. No service lights come on and there are no odd noises.

Now what? I need to take it back, and I really could use some advice on what to tell them to check next. Starting with the cheapest options first.

Thanks so much for your time!