This looks like it is exactly what I need and it appears to eliminate virtually the entire circuit I'm having trouble with.Freescale makes pressure sensors with a 5v supply that will simplify using with an Arduino.
On the downside, I'm trying to learn how to do this. Going with this component, and not figuring out what I'm doing wrong, seems like I'm giving up. I'd really like to figure out what is wrong with the circuit, make it work, then replace it with this self contained component. If I don't do that, I'm just going to make the same mistake some tome later.