The 50cc fuel tank appears to be the same for the 150 (schematic-wise), so you may want to follow the instructions here for troubleshooting (section 17-1):
http://www.49ccscoot.com/manuals/Kymco_Super8_50_Service_Manual.pdf I'm assuming it sometimes has/will go back? If so, from the symptoms, sounds like perhaps an intermittent short to ground on Blue/White wire, or an intermittent connection on the Yellow/White wire (or connector to the fuel sender)?
Amongst more unlikely things, I suppose these could go wrong:
1) sender itself could go bad,
2) float springs a leak (so it just doesn't "float" anymore),
3) instrument gauge itself gone bad/bad connection.
Good luck, and let us know what it turned out to be.
Cheers