It definitely sounds like the fuel sender is having issues. At first, I always think "electrical" but being an MQ I'm tipping that there isn't a hell of a lot of electrical stuff on it! I could be wrong..
I don't know too much about fuel sender/floats, but I picture it a bit like the float in your toilet sistern.

Fuel Sender Explained
Showing the fitting of a fuel sender
Explains removal
More installations/replacements

Hope that helps a bit.