Did someone add an electric fuel pump, like a Mr. Gasket pump from an auto parts store..?? A PO did on my 450 w/Kohler power, and had similar problems. Tried a couple news ones, and was good for a while, but then acted up again. Apparently those little pumps get so hot, they will stop pumping fuel. Not sure if the little oscillating piston in them binds or what, but you can tell more by the sound when they do it. Even went as far as buying a pressure regulator w/return line, and still had the same problem.
I bought a new OEM manual fuel pump for it, and never had the problem again.
It's much easier to install with the engine out. And it's not that bad of a job to take it out. I just cribbed up, and had a section of plywood on top of the cribbing to slide it out on, and worked on it there. It was about a two hour job, in & out, and I took a couple coffee & smoke breaks.
This was about 4 years ago, and OEM fuel pumps then were right at $52.00. Money well spent.