06 corolla s mechanical/mileage question

i'm hoping someone here might have some idea on this, as i'm not really sure where else to go... i somewhat recently bought an 06 toyota corolla s, and right as i pulled it out of the lot of the dealership, it almost stalled out twice, but never fully did. i thought it was just because it was very cold that day, and the vehicle hadn't run for a while, and there were no real other problems, so i let it go. i began to notice that it idled kind of hard/choppy (for lack of a better way to describe it) when i would roll up to a stop sign or just sit with my foot on the break at lights, and my gas mileage is definitely below the advertised 28 city/37 highway (i figured one time that i was getting about 19 city). i took it back to the dealership and complained, and they told me the mileage problem was just because it was cold outside, and the idle was just 'road feel' and they couldn't do anything without a light on the dash. well, it's warmer now, and the idle seems rougher if anything, but there is still no check engine light coming on. still poor mileage. my warranty expired, and i don't particularly want to go back to them, since they weren't very helpful anyway, but i am paying an arm & a leg for gas, and i'd like to get this fixed if i can. anyone experienced anything like this?

* they also mentioned they switched out the computer right before they gave it to me, but i've driven it about 2500 miles since then, so i've been told it shouldn't be an issue.