I've had some problems with my 85 El Camino with a 305 running rough. I found that my EGR valve isn't working. It get it's vacuum via a thermal vacuum switch between the carb and the EGR. There is a 2 pin connector that has 12 volts and what I assume is a ground wire. I can see vacuum changes at the carb, so I think the TVS is bad. My question is how does this thermal switch work? How does it sense when to open and allow vacuum to activate the EGR valve.

Most of the newer EGR valve inputs use a relay type operation. Input for them is driven by the computer. Temperature amoung other things controls the relay operation. Have you checked the error codes from the computer? Usually if something happens in the area you should see only a single error code coming back.
Found sometimes it's a vaccum line that is causing the problem. If you do suspect the EGR control relay, replace it with a new one. Put on a junkyard one once and it worked worse. Drove me crazy 'cause I didn't know what else could be wrong. Figured that it couldn't be the part I just changed.

Thanks for responding John. I started by making sure that the EGR valve could move. Then I checked for vacuum on the output side of the TVS, there was nothing. That is why I suspect the switch is bad.