Is my hot water heater going out??

possible bad power transformer or unit is overheating

Unless you are using a tankless hot water heater, there is a limited amount of hot water available in the tank. Electric and gas water tanks need time to recover after use, and if you are using heavily, it can exhaust the available supply. That said, there are a couple reasons why you may have less hot water than seems normal. In both electric and gas water heaters, silt can form in the bottom of the tank. This will build up over time and displace water. It also reduces the heating efficiency of gas units. There is a drain valve on the bottom of water heater tanks to allow for purging of this silt periodically, but few ever do it. You could also have, in the case of an electric hot water heater, a bad element or thermostat. Most electric heaters have an upper and lower element and thermostat. If one fails the other will still work, but the water takes longer to get hot and will deplete faster.

There is an adjustment knob to control how hot the water can become. There might be a problem at the tap (however, if this is the case, you would still get hot water everywhere in the house). If the entire house isn't getting hot water, and the knob is set at the hottest water, you probably need to get the hot water tank replaced.

It *may* not be a problem with the water heater itself necessarily. When you say "keep adjusting the water temperature when you're taking a shower" I presume you're talking about turning the shower hot/cold knob(s) and not the temp thermostat on the water heater itself.
If so, when it starts to run colder, turn the knob all the way to full hot. Is the water then very hot? If so, the water heater is working fine & doing its job, and the problem is likely to be in the shower faucet mixing valve itself. Sometimes a faulty one will expand the inner seals/cartridge as hot water flows thru it, and starts to (wrongly) restrict the hot water flow, thus causing the constant need to "turn the dial up a notch".

You could start by seeing if the thermostat is wide open. Sounds like it is on low or vacay mode. How far is the run between your heater and the bathing area. Might need to insualet some piping.

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if it has been constant from instalation it might be that ur shower head is rated too low to get enogh gpm ( gallons per minute) it will tell u most of the time around the shower head where it threads onto the spot of the wall what the gpm is. alot of the energy saver heads are restricting tehgpm to 1.5 or .5 gpms to conserve water well the water heater will only operate correctally around .5 gpm depending on model so if u are not pulling enough water u will not turn the water heater on to get ur hot water all the time so go to lowes ( or locally trusted hardware) and get a new shower head make shure u look for the gpm on teh head and check in ur owners manual to see what gpm ur water heater kicks on

P.S. if ur shower head is rated at higher gpm say 1.5 and it is corroded or pluged up say with lime it is going to restrict it causing a lower gpm

I had the EXACT same problem (installed my tankless last winter and worked fine, now this summer it fluctuates, have to turn the shower completely off and then on again to get any hot water and then it only lasts for a couple of minutes!) Argh - it is frustrating!

Here's what I found: the demand for hot water is lower during the summer (hot) months because the "cold" water coming in is already quite warm. You don't need as much hot water to make the shower temp comfortable. SO, my tankless was shutting off due to low flow demand. I've removed the flow restrictor from my shower head and it helped alot but didn't completely solve the prob. Next step is what has been recommended to me by several people but is much more expensive - change the pressure valve on your shower to a temperature valve. Or just wait til winter and it should work ok again :)

these units need lots of flow to start and run. Set the temp on the unit lower and the people in the shower will have to use more hot and less cold which will make the unit more stable.

You can eliminate the shower control knob possibility by turning on your hot water anywhere else in the house. If you get hot water in other places, then it could be the knob causing it. A substantial lime buildup in the tank can also cause poor heat transfer from the elements to the water.

if your heater is near the ten year mark, thats signs of going bad.....