Your fuel pump is shot. If it's an original pump, rebuild it. If it isn't, buy an original pump and rebuild that one. New pumps are not worth installing at any price.
Make sure your fuel pressure doesn't exceed 4psi.
My truck has all steel fuel lines and I live in Atlanta, GA where it's 90+ all summer long. The fuel doesn't get too hot. I use regular 87 deathanol. I also use only a mechanical fuel pump with no electric pump installed.
What did you discover about your manifold flapper? Is it working or not? If it isn't, your engine will never operate correctly.
It's hot under the hood of a car. 125 on a fuel line doesn't seem that bad.
