1999 Camry EGR mystery

For months I keep getting a P0401 code on my 1999 4 Cyl Camry. The car has 191,000 miles on it and still starts and runs like new.

I first removed and cleaned the EGR valve which was heavily caked with carbon. I thoroughly cleaned it and checked the vacuum diaphragm for it ability to hold vacuum. I cleaned all the manifold ports from carbon build up and checked and cleaned all the manifold vacuum ports and hoses. I also checked all the other related valve components and found them to be working fine.

I then reassembled the EGR system and almost immediately the check engine light went out all by itself and stayed off for nearly two months when suddenly the check engine light reappeared and again was giving the same P0401 code.

I started investigating and found that if I turned off the engine and reached back and manually moved the vacuum diaphragm up and down with my fingers that the diaphragm seemed to move up and down freely without sticking. With the engine warmed up and running I found that if I manually move the diaphragm up with my fingers, even slightly, the engine would immediately stall out. I then noticed after doing this regiment of manually moving the diaphragm up and down that the next time I would drive the car the engine light would go back off and stay off for a month or two. When it finally came back on again I would do the same steps of manually moving the diaphragm and the check engine light would go back off again for several months. I have done this several times now and each time the check engine light will automatically go back off and then back on after a couple of months.
I have not replaced the EGR valve, but is this repeated experience with the EGR valve itself a good indicator that IT IS the EGR valve that is causing the problem?