How do you have the sending unit mounted? Straight to the fuel filter housing? That's sure to kill the sender in a month, if it hasn't already done damage. Engine will rattle it around creating bouncing and after a while, it won't bounce anymore and just read max i experienced this with glow shift.
Should have a relocation cable, and be mounted away from engine vibration for maximum life.
If the above, is already done, did you inspect your fuel test port for debris before installing? Bottom of fuel housing clean? Blue spring mod done?
I have my sending unit mounted like this 2005 Ford F250 4x2 - Fuel pressure gauge There are all kind of sensors mounted directly on this engine and felt this shouldn't make a difference for this one either. I got roughly 5 dollars in these adapters. Where as it's 50.00 to 60.00 bucks from different vendors plus shipping!!! Im going to call Isspro tomorrow. Did some searching online and there might be an upgraded sender.