Probe Fuel Gauage Issues

I have replaced my fuel sending unit twice, and I'm back to the one that I bought my car with but I cannot get it to read accurately. When it's full, it's fine but when it's empty sometimes it will stick at 1/2 full and others after about 150 miles it'll read empty. I keep track of my gas by the mileage i.e. 350 miles it's going to be empty soon. I can drop gas tank and take the sending unit out and manually move the float and it'll read just fine and I've cleaned out my gas tank. Any ideas?