fuel gauge going crazy

Probably the sending unit is dying, or you have a loss of electrical ground. You can test the dash gauge with a meter. You could also test the sending unit but as that is likely part of the fuel pump assembly, you'd have to drop the tank to get to it. (check the floor pan area above the tank for an access panel first.) You can google instructions on how to test them. FYI a sending unit can pass an electrical test and still be bad though, if the float on it is detached or rotted away. If you do have to drop tank to get to it, you might as well budget for replacing the fuel pump.