Shower temp keeps changing
I have a 50 year old single level house (just moved in), that was added on to twice. I found supplemental water heaters in each of the three bathroom closets. The original water heater is a good ways away so I think this is what they did rather than increase the size of the heater. The more-or-less instant-on hot water is a nice touch. However, in one of the showers, the hot water will be comfortable for about a minute then go luke cold for about ten seconds, then back to original temp for about a minute, then the cycle repeats itself. You can almost set your watch to the timing of the cycle, which tells me this is a solvable mystery. Being a shade tree home improvement guy has brought no answers. I have replaced the cartridge to no avail. Any ideas?