Well, I spent a few hours on it today. I could only get the F code to show up in reverse but not at all times. All I have to do is touch the accelerator in reverse, hear the left contact pull in, and then release the pedal. About 50% of the time F code would pop up and shut down the machine. As Jplayer and mrfixit suggested, I checked the fingers in the logic connector and had a quick look at the accelerator. I swapped the direction contactors around, thinking that maybe the double acting contacts were not making/breaking at the right time, then on to trying a different logic unit, swapping the diodes and transistors around without any luck.

I did find the left drive motor had an small short to ground at the field coil terminal so I pulled it and sent it out to the motor shop for inspection. Hope that cures the problem but I'm not convinced.