How come the minute you fix one thing on these cars something else goes wrong? LOL
Now I've got a gas gauge that swings from full to completely empty and then maybe half way for awhile etc. What's the most likely cause? a Bad float in the tank? Sending Unit?

 

Bad grounds or bad connections. Typically the sender grounds through its body to the sender
plate and from there via the tank. I had to add a jump wire from the sender unit as it comes
through the sender mount plate to one of the screws holding the mount plate in. Fixed.

 

flaky sending unit. i have had good luck with fuel system cleaners, oddly

 

iirc the electric connectors for the fuel sending unit is exposed right below where you pour the gas in. maybe splashing into a water puddle kicked up some water into an exposed area of the connection between harness and fuel sending unit.

hopefully you can check that so you don't have to drop the tank and check the fuel sending unit itself.