Replacing the sender is an absolute bear to do, doable but a real chore. For your case I hope it is the gauge. These gauges are known to not be that reliable. I agree to use a multimeter and check your ground.
The other issue is the sender itself has a ground wire coming off the sender at top of the tank and when I replaced my sender gasket the ground wire and wire connections were grimy because of the fuel leak I had. After you test your gauge I would then suspect those connections. Finally I would suspect the sender unit itself. In that order.
Do a search on my posts and you’ll find about 18 months ago I replaced my sender gasket and have a lot of detail there if you have to go that route. You can always PM me and I’ll be happy to help point you in the right direction with tips and discussion of how to get in there.