New Hot Water Heater, but still erratic temp

I recently bought a new 40 gal tank (gas heated). I have the same problem as with the old tank. In my bathroom somedays I can have the shower level at "H" and the the water will be pretty warm, but not Hot. Other days that same temperature is achieved by setting the shower dial appx a 1/4 up from "H".

I do not have this problem with my faucets just for the shower. I had the same problem prior to getting the old tank, but I was told I needed a new tank. However, I do admit that previously to the new tank if the level were at "H" it was barely warm on SOME days. Other days it was fine.

Why the problem with the shower. The temp is better with the new tank, but not what I expected and it definitely is erractic.
Unhappy

Shower Faucet Problem

It is possible that the original problem is with the shower faucet itself and not with the water heater
Did the plumber troubleshoot the problem before the new 40 gallon heater was installed?
Be careful when adjusting the water heater temperature control not to raise it too high to cause burns until the problem is corrected.

It sounds like you may have a pressure-balanced type shower valve, and the balancing spool may be stuck. This can be disassembled for cleaning and lubrication.

The main thing that goes wrong with water heaters is they leak. SLow recovery can be due to mineral build-up. Fluctuating temp. is not a common symptom.