Most likely suspect is float, the people with intermittent fuel level readings, your float is probably getting stuck and then breaking loose at your respective readings...if that makes sense.
The following would be very difficult, dropping the tank etc, but will tell you for sure. Although if you are going to go this far, you may as well replace the float/sending unit while the tank is down. You can check the float by manually attempting to raise or lower it, and check the gauge. If the gauge doesn't move, then you know the gauges are broken. If the gauges move then you know it is the float. Rather than do all that, start with the least expensive replacement part, maybe a diff gauge from another car you know works. I would normally say its the float right away, but the gauges have been an issue for many stang owners. Gauges can be found pretty cheap so I would start there.