Any sound of water sloshing in the float? Get a pan of very hot water and 
drop in the float. The heat will expand any air inside and force it out the 
cracks, just watch for the bubbles. Sounds like it's just sticking though, 
since it's jumping once the reading is far enough from the real water level. 
Mazolla time?
Is there a wiper arm type sender that's dying? tim s.