fuel guage will go to 3/4 full and gradually fall to about 1/8 tank.

It sounds like you have a fuel level sender/fuel pump module failure. The fuel level sender and fuel pump module are one unit and is located inside the fuel tank. There is a slight possibility it is a gauge or electrical connector issue but typically if there is a gauge issue, the whole cluster will act up (i.e. speedometer and temperature gauge stop functioning). The best thing to do is to have testing done to verify.