A shower temperature that spontaneously fluctuates between cold and hot cycles is typically caused by fluctuations in household water pressure interacting with a failing pressure-balancing cartridge in the mixing valve. When someone flushes a toilet or runs a washing machine, the cold water pressure drops; a worn pressure-balancing spool fails to compensate properly, causing the water to surge hot, then cold as pressures restabilize. Verification: Replace the worn mixing valve cartridge with an exact-matching new unit.
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