1991 Geo Prizm slowly dieing.

I bought her cheap. Ran like a champ from CO to CA. Also from CA to OH. Burns oil but kept up with it. Have changed it 3x since April and all the driving has been since then. Once back in Ohio it started to act like it was missing after it warmed up. Cold it ran at a slightly higher idle (normal) but once it warmed up it started the missing and power loss. My mileage dropped almost in half down from 35ish mpg. I stalled it the other day in a dead end doing a 3 pt turn. Went to start it and it starts fine but quickly idles down to a stop. If I try to gas it on the start it sounds like I'm somehow flooding it and it dies much faster. Starts first crank but then just idles down to dead with in 2-3 seconds. It turns over instantly but also instantly runs down to a stop. Any ideas? Since it was running I was thinking some sort of sensor. I had taken it by a few shops but it needed an older computer to trouble shoot. Found one finally but he couldn't get it to read the car. I only need her as a commuter 3 miles. Thanks.