We have a new gas water heater that was working fine after installation. Now when we turn on the hot water, it will be scalding hot for a few seconds, then run cold until we turn the faucet off and back on, then it will be hot for a few more seconds until it runs cold. It does this on every faucet in the house.

Angie,

Please check the gas hot water heather control for proper setting

Itsure sounds like the installer applied too much heat to the cold water inlet melting the flare off the plastic dip tube allowing it to fall into the tank. A dip tube is a plastic tube in your water heater that directs the incoming cold water past the hot water in the upper part of the tank down to the bottom where it will be heated and raise to the top. If it's gone then you would get hot water until cold water entered the tank. The water coming out of the showerwill then turn cold. You turn the water off and the hot water raises to the top until you make a draw and the whole thing starts over. The solution is to shut off the water to the heater and turn the control to "Pilot". Remove the fittings from the cold water inlet and, if you can, fish the old dip tube out and replace it. Don't worry too much if you can't retrieve it. It's not hurting anything by being there. If you don't flush your heater on a regular schedule let me show you how.
For long life and fewer troubles you should keep your heater clear of mineral build-up by FLUSHING NOT DRAINING on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear. Now flush out your hot water lines on ALL fixtures that are affected . Now pull each aerator and clean the screens. Be sure you put them back togather the same way you took them out. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps, Tom

Hey Tom... this time I waited cause I knew I was behind you by a few minutes!

I agree that this is most likely the dip tube (see pics. Below).

Here ia a visual for AG... so they can see what we are talking about!

Good day all!

I agree with sppedball1 and massplumber, please Click on the following link it may help you too,

How a water heater dip tube works and what it looks like.