[bmajvi]

Now that you know the engine runs fine on an external tank, identifying the core issue is a pretty straightforward process of elimination. An easy way to test if the starvation is caused by a blocked fuel tank vent is to just remove the internal gas tank fill cap while underway, and see if it "solves" the issue. If no change, maybe Uboo's suggestion of a new bulb would be worth trying. I know you said you cleaned the tank out really well not long ago, but I think it's still possible that something is getting clogged in the pickup tube (probably because my cousin had the same exact problems and symptoms not long ago, and he found algae clogging up the mesh on the pickup tube). He actually took OFF the mesh from the pickup tube (after cleaning the tank and refilling with fresh fuel), as he has a clear bowl Racor water separator/filter, figuring that it would be better to capture crud in the bowl where you can see it, rather than trapping it invisibly inside the tank.