Have a relatively new Bradford White 50 Gal gas hot water heater. Sometimes when there has been NO recent use the water will be HOT. Other times under the same conditions of no previous use it is cool or becomes cool VERY quickly. The heater clearly can produce LOTS of hot water some times, but not others . Temp setting is medium. Sometimes 2 people can each take 15 minute showers back to back, but . Other times with no previous use one person cannot get 10 minutes out of it. Any ideas what's going on?

Having less hot water than usual can be caused by several things. It could be a burned out element, leaving only one to work. It could be a defective thermostat. It could be a build up of sediment at the bottom of the tank, although that wouldn't cause a sudden change in the amount of cold water. That happens over months and years. I suspect it is more likely a broken dip tube. There is a tube that sends incoming cold water down to the bottom of the tank. The hot water, which naturally rises to the top, goes right out the top hot water line. The dip tube can corrode and fall off inside the tank. Then incoming cold water doesn't make it to the bottom and the hot water at the top gets quickly mixed with cold water. The result is hot water followed quickly by lukewarm and then cold water. The dip tube can be replaced and the water heater restored in most cases. See a plumbing supply house for a replacement part.

Agree with lots that Jason presented (except the burned out element... not present on a gas water heater), but I am having a tendency to think you may be having an issue with the gas valve/thermostat itself with the information you presented.

How old is this gas water heater? Let us know...

Mark