I find this odd, but someone else may be experiencing it. Whenever I take a shower, the water often changes temperature... sometimes hot, sometimes cold. It never lasts too long, usually long enough just to notice it, then it returns. But it will happen 5-10 times during a shower. The obvious answer is that someone is running water while I shower, but it happens when I or my wife are alone in the home. Seems a bit peculiar to me, but I can't find any reason for ti to be happening. And it's not just the shower. If I am washing my hands for an extended amount of time the water temp fluctuates as well.

Is there a recirculating pump on your hot water system? Let me know, Tom

Nope, no pump A few more details, it is electric, and 80 gallon. And we've never run out of hot water.

How old is the heater?

Tom can shoot me down on this, but consider a possible defective or disintergrated dip tube.

Very important question here. Do you notice any difference in pressure when this happens?

Home was built in 2003... and I do not notice any difference in pressure.

Tom hasn't come along to shoot me down yet so I'm going to say that your dip tub is gone. Cold water is coming in at the top rather than the bottom of the water heater and going directly out the hot water outlet.
Why this is speratic probably has to do with the mixing of hot and cold water and circulation within the tank

One way to determine a dip tube problem is to remove your faucet aerators and look for small plastic particles clogging the screen. To check whether these particles are plastic or merely calcium, put some in a capful of vinegar. If they do not dissolve, they are plastic and you have a defective dip tube. (Caution: this is only one way to test, and a lack of particles may not mean that your dip tube is functioning properly).

Also please open the link below it will show how the dip tube works and also will show what happens when it fails, Good Luck.

John

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