This sounds like the dip tube is broken off. There was a problem with dip tubes a number of years ago, but that was a manufacturing goof and was corrected. However, the symptoms sure sound like dip tube. FYI, a dip tube is a plastic tube that fits under the cold water intake and conducts the incoming cold water to the bottom of the tank. The burner is of course at the bottom of the tank and since hot water rises, the entire tank will have hot water. Without the tube, only the top portion of the tank will be hot and therefore you will run out of hot water quickly. It's easy to check the tube, just remove the intake nipple on top of the tank and with your pinky finger find the flared top of the tube. Fish the tube out and see if it is intact. If not, get a new tube, trim the length if it is too long for your tank, replace everything and you should be good to go. If that's not the problem, then I don't have a better guess.