Is there a chance that the float is hanging up when filled, then after the fuel drops enough, the float gets "unstuck"? Maybe vibration or a bump in the road helping it out? Just a thought. Unless there is an issue with the new sending unit? You can pull the sending out and check with an ohm meter to see if it reads in all positions. And while your at it, look for other issues like hangups, and stuck movement of the float in the full position. Not to mention tank clearance....
