Bongo Fury

Does sound like air in the system problem.
As others have said - pump problems.
After a good run, the tickover speed is dropping when throttle is released, you are having to catch it until it settles again.
You could try increasing tick over speed and when it starts to drop, it will get to a position where it is still too fast to cut out.
This is a temp repair.
Other item could be solenoid on its way out.
Get the torch out and check all pipes and connections for leaks.

If you have seen the posts on re earthing the tank

im sure it comes out with the fuel gauge sender

plate in floor behind drivers seat