Was battling a similar scenario here in nebraski..... Just recently.
Its just about has to be a fuel issue.
Low miles and stored with fuel..... Just about promises problems.
Injectors on this one stuck open. Fuel pump was full of garbage. Tank looked good and clean.
Mechanic and owner decided to get new injectors. Didn't clean tank. Pump souned terrible, and it wasn't cleaned out.
Immediately plugged new injectors.
Convinced them to get new injectors, clean out tank and pump. They bought a new pump, and ran some sea foam thru it.
Appears now to running fine.
What i've seen on these crapped up pumps is usually a slimy yellow mess. Appears to be the liner breaking down.... But i don't know that. Even though with a good lite the tank looks clean.
Depending on how old the fuel, ive seen it actually crystalized on the screens.
I also tell all new owners NOT to start their new purchase until its been thoroughly cleaned regarding fuel. It just creates so much more work.
Hope that gives you some ideas...
Sorry for the late answer, but some of these posts get hidden immediately if someone posts something new...
Bruce