air in the hot water lines??

I had a new electric hot water heater installed in my house about 5 years ago. Has worked fine no problems until about the last two months. Now when you turn the hot water on at first it spits and sputters like it does after the water has been shut off and has air in the lines. The temp in the shower is very hard to maintain now too. You get it set just perfect be showering away and it will get really really hot then go really cold and back again without anyone touching any other water supply in the house.

What is the life span of an electric water heater? Any ideas on what could be wrong with this silly thing? We have two adults two teenagers and two toddlers in our house of one bathroom if that matters any...

Do you have an expansion tank on the water heater line? Are you on private well or public water?
If you have an expansion tank or are on a private well, you could have a tank bladder leak (not the water heater), and either the expansion tank or the pressure tank for the pump needs to be replaced due to a leaking bladder.
If you're on public water, air also can get in the lines from nearby tap-ons, work on the system, etc.
Try running the hot water only through a tub spout until all of the air is out of the water heater and it stops spitting air.
If that doesn't work, check out the above. Air is getting into the water heater from some source.
Good luck!
Mike