I have been chasing this problem for two years and have finally resolved it.
The above list is the basic three problems. I have had help from JPI, David Fletcher, Bill Scott, Greg Harrison (3Y3) and others from the Grumman Gang. The problem may well exist in other aircraft as well. The final problem was a tiny pin hole air leak on the fuel line that runs from the selector valve to the firewall inside the cabin. This leak was so small that there was no sign of fuel stains or other indications on the outside of the line. It appears that the fuel line rubbed against the heater duct about 2" from the fire wall. This started a corrosion site that continued until there was a hole that allowed air to be sucked into the fuel line. See the pictures below.
Fuel line from Fuel Selector to firewall
Fuel line from Fuel Selector to firewall showing small pinhole
The first indication of a problem was three or more years ago when I would occasionally get a smell of fuel when landing. Then the next indication came when the JPI 450 fuel flow instrument would not read correctly. JPI told me that it was due to air bubbles in the fuel system. I visually checked all the fittings for fuel stains and then had JPI send me a new head and a new transducer. Neither fixed the problem.
Then last week I went to fly and noticed that the fuel pressure was surging, from 5 lbs to almost 0. I didn't fly and figured that it was time to really find out what was wrong. So I replaced the fuel selector valve as this is what many people suggested and it seemed the easiest to do. However this didn't fix anything. We decided to work on the plane over the holidays so I called Flethair to get an electric fuel pump. The service guy said wait a minute and you can talk to David Fletcher. David told me I didn't need an electric fuel pump, the problem is very likely to be a bad fitting causing an air leak on the aluminum fuel line at the fire wall.
So I went out today to remove the fuel line to inspect it. And you can see what I found. David Fletcher was exactly on target. Replacing the fuel line resolved all of the above problems. No fuel smell, the highest and steadiest fuel pressure I have ever seen on my plane, and the fuel flow reading is steady and accurate.
Thanks to David Fletcher, he saved me from buying an electric fuel pump that I didn't need, and to everyone else on the Grumman Gang that helped.
Everyone should check their fuel line as this was caused by the heater duct contacting the fuel line. This is similar to the problems with the defrost duct wearing on the oil line. Check it if you have any indication of fuel smell or low fuel pressure.
I plan on replacing all the remaining aluminum fuel lines inside the cabin at the annual in January.