Is it pressure fluctuation or hot to temperature fluctuation? Is anybody running water elsewhere in the house while you're in the shower? If it is a temp. fluctuation and no one is running water while you're in the shower your dip tube could be bad in your water heater.
The dip tube is a tube that extends from the cold water inlet to of your water heater to the bottom of the tank. It forces the cold water to the bottom of the tank which forces the hot water out the top. If your dip tube is beginning to deteriorate and has developed a hole near the top or is just gone, the cold watr could go in the cold side and straigh out the and straight out the hot side. But, this is just one idea. There are other things it could be. I would need a few more details (see the questions I asked in my first response).