I think I may have figured it out!!
Being that I'm a software guy I never am really concerned for these EE type issues. I got in all my sensors and was thinking of hooking them up when I started to think about the power they draw.
I believe I may be drawing too much power out of the 5V power onboard the arduino causing it to loose memory/crash. (see: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1178565105/4#4
Apparently it has a ~400mA current limit (http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1183268436/2#2
In the current setup I am powering a serial LCD (I believe it draws ~60mA with the backlight and an accelerometer which should be using only 0.6 mA). So I'm not sure how this is the problem. It could also be a noisy power supply from the car. Cars have a problem delivering a nice even 12V. Sometimes its 13something and sometimes under load it can be lower.
All the sensors I'd like to power off the 5V:
1 accelerometer drawing 0.6mA
2 buttons drawing 2mA each (using 2.2kOhm resistors...set to HIGH normally open)
2 thermistors in voltage dividers using 2.5kohm resistors providing 790-18680Ohm's themselves ???draw each??
1 pressure sensor drawing 0.5mA
This should all work correctly right (we're way far from 500mA)?
What about the voltage dividers?? How much do they draw/how do i calculate that?