Fluctuating temperature often starts with changing flow
A shower can only hold a steady temperature if the hot and cold supplies remain reasonably stable. When somebody flushes a toilet, opens another tap or starts an appliance, the balance can change. In a well-matched thermostatic shower the valve should compensate within limits, but a weak or poorly balanced supply can still produce noticeable swings. Note whether the problem happens only when other water is being used. If the temperature varies even when the shower has the whole supply to itself, the shower valve or hot-water source deserves closer attention.
Thermostatic cartridges can wear or become scaled
Inside a thermostatic mixer, a cartridge responds to temperature and adjusts the blend. Scale, debris or wear can make it slow, sticky or inaccurate. The shower may start hot, then cool abruptly, or require constant adjustment to stay comfortable. Cleaning inlet filters may help where debris is the problem, but cartridge servicing depends on the design. Do not dismantle a thermostatic control without knowing how it is calibrated; incorrect reassembly can affect the safe maximum temperature.
Combi boilers can create their own pattern
A combination boiler heats water as it is drawn, so very low or unstable flow can cause the burner to cycle. The shower may then alternate between warmer and cooler water even though the mixer valve is sound. If hot taps elsewhere also fluctuate, the boiler or hot-water side becomes more likely. A boiler fault needs the appropriate heating engineer rather than ordinary shower servicing. Record whether the boiler display changes or shows a fault when the temperature swings.
A partially blocked shower can upset the balance
Restricted filters, a scaled head or a damaged hose can reduce flow enough to affect the way the shower and boiler behave. This is especially noticeable on systems that need a minimum flow rate. Clean obvious removable parts first and compare the shower with another hot outlet. If the shower flow is substantially lower than the basin tap, the restriction may be local. If every hot outlet behaves badly, the investigation needs to move upstream.
Safety matters more than comfort alone
Sudden very hot water can scald, particularly children and older people. If the shower is producing unpredictable high temperatures, do not keep using it while trying to find a setting that happens to work. Thermostatic safety features should be treated as part of the repair, not a convenience. A plumber can test the supply temperatures and the valve response; a heating engineer may be needed if the hot-water appliance is responsible. The right trade depends on where the fluctuation begins.
Information that helps before a visit
Say whether the shower is thermostatic, electric or a simple mixer. Explain whether temperature changes coincide with other taps being used and whether hot water at sinks is stable. Mention the boiler or cylinder type and any recent work on the system. If the shower was once reliable and has gradually deteriorated, say roughly when you noticed the change. These details help separate a supply problem from a worn shower component before parts are ordered.
Try to reproduce the temperature change before the visit
Intermittent faults are much easier to diagnose when you know what triggers them. Run the shower at its usual setting and note how long it takes before the temperature changes. If another person can safely open a basin tap or flush a toilet, observe whether that causes the swing. Check whether the same hot-and-cold pattern occurs at an ordinary hot tap. Do not keep testing if the water becomes dangerously hot. Write down the boiler display or any fault code if it changes at the same time. These simple observations can separate a shower-valve problem from a wider hot-water issue. They also help avoid replacing a thermostatic cartridge when the real cause is a boiler cycling because the flow rate is too low.
This page is general guidance about why does my shower keep going hot and cold; the cause in an individual property depends on the actual plumbing arrangement.