Fixed. I'll give a complete rundown here, since this code 31 on a Fox Mustang seems to be a popular Google search, as well as in the F-150/Bronco community, so maybe this will help others in the future.
The Sunpro scanner book says Code 31 - EGR valve position (EVP) sensor or pressure feedback EGR (PFE) sensor - signal voltage is below minimum specification or EGR vacuum regulator (EVR) solenoid circuit problems.
I took MFE's suggestion and pulled the 10 pin connectors apart, (salt and pepper plugs on the back center of the intake) cleaned them up with electronic cleaner spray, and applied dielectric grease to all pins before plugging them back in. Test drive: No changes. Next, check the resistance range of the EPS. Removed it from the EGR valve and probed the connections. Around 0-4 ohms throughout the pin travel, linear action. Tested another one I have, identical readings, no problem there. Next, check EVP voltage, key on: 4.5 volts - in range. Next, check EGR vacuum from the vacuum regulator solenoid, engine running. You are not supposed to feel any vacuum at idle, but I did - a little. I learned of a foam filter in these units from a Bronco site, they get dirty and plugged. I pried the cap off the top and pulled the filter out. It was packed with dirt and just about solid, so I soaked it in water and dish soap, cleaned it up, rinsed and dried it. I also blew the dust out of the unit with compressed air, and hit it with a shop vac for good measure.
I also had to replace three plastic vacuum hoses on the tree, because they get brittle with age and all split when I unplugged the tree including two right at the rubber. Had to drill them out. Fortunately, I have a pile of these hoses from boneyard Tauruses and such, so I was able to get it all back together and looking decent - albeit with a few different color hoses.
Test drive with the cleaned vacuum solenoid filter - no CEL. None. I now have two days and four errand cycles on it, no CEL. Seems it's fixed. The plugged filter didn't allow the diaphragm to move enough to supply vacuum to the EGR valve, and the EVP reported insufficient movement to the EEC and lit the CEL. It would also explain the foot on/foot off gas CEL on/off. So while the code didn't tell exactly what to look for, it did provide an area to look in.