need help from gage guru's please

It's nice when everything works!!
Just a stabb - Even though replaced 10 years, it sound to me like your "sending unit" is acting up. You say when it's full..it reads full, however after about 8 gallions use or so it reads MT. My guess is that if your sending unit is a float design the float arm is loosing contact with the "resistance wire" (in the contact shoe box in the tank). When the gas drops it gives you a wrong indication or no indication at all (MT). Firstly, your getting your 12 volts at the tank because your guage shows full at full. Take the connection contact off the tank and the guage should drop to MT. Now before you go ahead and rip out the sending unit...you could just get "any" old sender and dry connect the voltage wire to the contact. (ensure the unit itself is grounded) then move the float arm up/down while someome is watching your guage. If it works..your unit is hosed...if not, then yes possible guage problem (ripping out the cluster sucks)