The shower suddenly stopped and then continued in short spurts

your not making sense with you question --state it goes cold in middle of shower --but state it will stay hot or cold forever ----if its the latter continued hot till you shut off water that normal working if the unit shuts down during shower then you have a problem ----unit is set at one temp and runs at that temp and you use shower 1 handle / 2 valve what ever you own to adjust temp their are really only 4 reasons stops making hot water ----2 are usually well water supply --incoming water pressure /flow or to cold ----other are gas valve -or sensor so i would look into your shower handles since almost all have adjustments and anti scold safety

The rubber seat on the end of the control may be lose.

Screw off shower head-obstruction in water saver disc or screen,piece of rubber or rust

This is PROBABLY due to a bad volume control pot or wiring to the pot. Sometimes people have used DeOxit or Blue shower in the pots and it wrecks them. The resistance element may be cracked in he pot. The break would be on the side of the pot that connects to ground.

gotta cut open the wall and take a look---you don't want mold and water continuously present inside your walls---think creeping death!

sounds like a boad problem or a relay playing up.

I had the same problem and when i researched the problem on the internet it was suggested I buy a new Solenoid, which can be bought from the net for about £17.00. I fitted it myself and the shower has carried on working for about 4mths but has now stopped working again so now looking at getting a new shower instead. IT was a good stop gap though and for that price worth giving it a go

water pump clogged with debry clean out
or the your home lost preasure in pipes