A further update: I picked up the car on Sunday. It started up, and settled into a nice idle -- not the sputtering and stalling it did previously. I drove it home gingerly as suggested by the mechanic (he said that there was a lot of gas in the old oil). We then used it to go to lunch, and when I pulled away fromt he stop at the end of my street, my girlfriend turned to me with some surprise in her eyes and said "Wow, that's smooth."
I may have bitched about paying so much to get it going, but it is truly running better now than it ever has since I bought it.