My fuel gauge has started being very erratic. It was reading empty when I had half a tank, then it read full at half a tank and slowly came down to half a tank. I shook the truck when it read empty and that seemed to fix it. It's acting normal now but am I looking at electric or the actual sending unit. I have looked around a bit and if it was the sending unit it should be reading empty right? I don't think it got hung up as it was at half a tank and parked on level ground.