Tankless Supreme stops heating...?

Hello! I am new here and hoping for some useful advice. We have a S220, 14KW, 3 element heater. Recently, only while showering, (not when running water for a bath or anything else) the heater will stop heating. I watched it this morning while our son was showering and the red light was blinking while it was in use and then it just stopped and of course the water went cold. You have to shut the shower water off for a few seconds and then back on to get hot water again.... and it will likely cycle off again after maybe a 1 minute or 3. Can anyone give me some advice on this? WE have had it for years and it has been a GREAT heater, this is the only issue we have ever had with it.
This is a small house, 1200 sq ft. The water pressure is quite steady as there is a pressure regulator at the water pump.

Thank you!

Oh Boy where to start....
Everything about this sounds like just a total nightmare tankless installation...
I am truly surprised that you didn't have unsatisfactory performance from day one....

The unit is an off brand which is troubling right from the start. In fact I could only find this post here and 5 listed for sale on E-Bid for listings on a Google search. For some reason the only tankless units gas or, electric that seem to have any thing good for results seem to be from just a few major manufacturers....

With your listed location in MN. you are way outside of the area where electric tankless is truly a viable method of heating water. The electric units use a tremendous amount of electricity and only manage to show a small rate of rise at low flows so the only place they are popular is in the southern tier of states where the incoming water is almost hot already and not borderline ice cubes like you have in MN.

Now the beauty of it all. Try to find troubleshooting information and parts for this off brand unit....
Well at least the E-Bid listing gives a clue saying they are made by Advanced Tech International, Inc. of MIAMI, FL.
Sorry that is all the help I can provide....

Other than I would fix the unit with a replacement...

Supreme tankless water heater
6800 nw 37 ct
miami,fl.33147
ph-305-691-1111
fax-305-691-9354
web-supremetanklesswaterheater.com

I think this is a shower head not allowing enough GPM to flow through keeping the flow switch of the unit on. Change the shower head and I bet you will be fine