I'm not sure if I am an expert, but I took the MFI on my 72 E from totally screwed up to mostly in spec. There is not much air that runs thru the hose from the heat exchanger to the thermostat housing. If you can feel warm air on the exhaust side of the thermostat housing then it is getting enough air. I would first try cleaning the disks inside the housing and the housing itself to make sure the disks are not sticking. Complete directions can be found here:
http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm#MFI%20thermostat%20R%20&%20R