hey GA_Z, I had a similar problem in my 74 260z with carbs. The car sat for 5 years with half a tank of gas. I even got the gas tank stripped and re sealed. It did not help any...
I took a bicycle shift cable and pulled the center cable out of it and flared the end to make a little wire brush. I cramed it down the output tube on the gas tank until I knocked off the little screen on the end of the tube. I now run 2 fuel filters to catch any gunk that comes through. I had removed the fuel pump and reworked it but it was not coming on. I just ran a new wire with a switch on it so I could manually control it. Once the tank was clear and the pump was working I got excellent fuel pressure.
It is a carb system but your EFI could ave a similar problem.