In my opinion it has something to do with the fuel delivery somewhere blocked.
It already has been asked but did you remove the gas cap when it was running poor? You said it occurs after a while.
And try it without the fuel filter.
You never know!!
Is your fuel pump functioning at 100%. There is a test to check how much it can pump. Came across this info when I was looking for a fuel pump for my GL1200 LE.
85 Goldwing LTD
86 Goldwing Aspencade
Do you have the solid straw in addition to the filter/strainer in the fuel tank? Could the internals of the petcock contribute? Lol...Grasping at straws