I think it is the petcock. I have been getting a strong gas smell. Seams like it is coming from that area.
Maybe the filter in the tank. . . .
Getting a new filter at the same time would be a good idea.

Before jumping to that conclusion let's ask whether something has been added to the fuel line. Lengthening the fuel line can result in a bubble giving that symptom which is momentarily solved by adding a little more pressure going to reserve.
Willow is correct. If the fuel line is too long, it could be getting kinked and block the fuel.