Mine is about the same as well. When stopped it sits at the 1/2 mark and then when moving it drops back down. I've replaced the rad, thermostat, temp sending unit, water pump, fan clutch, and all hoses. All oem. Its been an expensive process. Just ordered new aftermarket guages though, hopefully the stock guages just suck.
But did you replace the FUEL gauge sender? Yes the FUEL gauge.
Do a search and see.