Working with Autometer to fix my problem, we've got it narrowed down to the sending unit. Since I couldn't find this information elsewhere, I wanted to share what their tech told me for troubleshooting the sending unit.

To check the sending unit, measure the resistance across from terminal to ground with the sending unit disconnected from the gauge. Below are the readings you should find.

0 psi 242 ohms +/- 17
25 psi 152 ohms +/- 12
50 psi 103 ohms +/- 10
75 psi 66 ohms +/- 9
100 psi 30.5 ohms +/- 5.5

Hope this helps in troubleshooting a sending unit problem.