Hello Im having trouble with my water not being at a consistent temperature. For example it starts scolding hot then quickly goes to luke warm. Once I shut it off and turn in back on it will get hot again. This is absolutely annoying when Im trying to shower. Please help. :confused:

I have two possible answers but neither are for sure the right answer.
One is that you have a two handle faucet one for hot and one for cold and these are the old type faucets with the rubber washers on the end of a stem that turns to turn the water on and off. In some cases the rubber washer is made of a material that swells in the heat therefore after a few minutes of hot water flowing over it it swells thereby shutting down the amount of hot water that can get by. When you turn it off it cools and then will let a larger amount of water through the faucet. The way to fix this problem is to repair the faucet with new washers and hope that was the problem.

Second if the first answer was way off base I have another answer that still may not be the answer but is just a suggestion. A water heater has a tube that sticks down into the water heater in the cold inlet side of the water heater it is called a dip tube. The function of this tube is to carry cold water to the botton of the water heater and that pushes the hot water up to the top of the tank therefore giving you lots of hot water before the water is mixed into warm water. There have been occasions where this plastic dip tube will deteriorate and become very short letting the cold water coming in the inlet to mix with the hot water at the top of the water heater. Therefore you water starts very hot but becomes warmer as the water mixes. The way to fix this problem is to replace the bad dip tube with a new one. One indication that this might be your problem is to have trouble with your aerators on you faucets stop over a period as this dip tube is deteriorating. If you have had this problen you should see small pieces of white plastic, maybe in the shape of very small squares, and very soft and easy to crush, showing up in your faucets.

I think Letmetellus second answer's right on target. A broken or faulty dip tube,(see image) will let hot water raise to the top of the tank for a hot draw. Then when you make a draw the cold water coming into the heater stays on top cooling off the water instead of going to the bottom as it should to be4 heated. That would be my guess. The dip tube is a white plastic located under the cold water inlet of the heater. Be advised that you may began to find little white particles of plastic, (These white pieces will be hard plastic) in your areator screens. Flush your heater and change out the dip rod. If you don't know how to flush your heater let me show you how.
For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing 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 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. 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

Can a dip tube be repaced by a DIY or better off a plumber?

Could it be just a dip tube or do you think I am better off replacing the HWH itself?
Can a dip tube be replaced by a DIY or am I better off letting plumber do it?

Easy job. Just disconnect the incoming cold water line. Often they have some sort of a union, something you don't have to turn the water heater to unscrew. Look for some sort of a nut. Once you have the line removed, you should be able to see the end of the dip tube. Pull out what is left, and drop the new one in. Put the feed line back together. Have a beer and then a nice hot shower.