Our issue was a bad RC. We swapped a loaner in this morning and everything has worked fine since. Our inability to reproduce the error made it really hard to diagnose, but we finally traced the problem to cold booting the RC (cold booting was causing the program to become corrupted).
Kevin - Thanks for taking the time to look at our code
We're now running a kit gyro with your gyro driver and a pot (which is used to control the direction our steerable front wheels are pointed).
Thanks for the help everyone, and watch out for defective RC's.