My shower temperature is unstable , keeps going hot and clod

An issue could cause the problem you're experiencing with your Aqualisa Quartz shower with the shower's thermostatic control valve. This valve regulates the water temperature and maintains a consistent temperature.
One common cause for a shower to become too hot is a malfunctioning thermostatic control valve. The valve may be stuck open, allowing hot water to flow through the shower even when the temperature is set to the lowest setting.
Another possible cause could be an issue with the shower's mixing valve, which is responsible for mixing the hot and cold water to achieve the desired temperature. If the mixing valve malfunctions, it could cause the shower to become too hot.
I recommend checking the thermostatic control valve and the mixing valve for any signs of damage or wear. If either of them is damaged, they will need to be replaced.
If the shower is still too hot even though you have set the temperature to the lowest, it might indicate that the thermostatic valve is not working correctly and needs to be replaced or repaired.
It's best to call a professional plumber or a service technician to check the shower and diagnose the problem; they will be able to find and fix the issue, ensuring that your shower will be safe and functional again.
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Do You know if there is an anti scald valve in the shower valve? If so it may be scald up with corrosion and need replaced! To see if this could be the problem, flush the toilet next to the shower while the shower is running. If temperature changes in shower I would say that is your problem! If You need to replace it, it would be cheaper to do it yourself! They should be videos on Youtube on how to do it! Good Luck! Regards!

You haven't provided much information to help narrow down the source of the problem.

Is the flow of both hot and cold water flowing freely? Are the water lines not clogged internally with corrosion or residue? Is it possible that the water is being diverted such as in an apartment building or to an outside spigot (sprinkler system) or kitchen faucets (dishwasher)?
Is your water heater temperature setting correct and is it supplying sufficient hot water flow to the shower?
Do you have a temperature balance cartridge in your shower faucet? If yes, it could possibly be faulty.

It *may* not be a problem with the water heater itself necessarily. When you say "keep adjusting the water temperature when you're taking a shower" I presume you're talking about turning the shower hot/cold knob(s) and not the temp thermostat on the water heater itself.
If so, when it starts to run colder, turn the knob all the way to full hot. Is the water then very hot? If so, the water heater is working fine & doing its job, and the problem is likely to be in the shower faucet mixing valve itself. Sometimes a faulty one will expand the inner seals/cartridge as hot water flows thru it, and starts to (wrongly) restrict the hot water flow, thus causing the constant need to "turn the dial up a notch".

It sounds like the cartridge has to be changed. Pressure balance is not working in the cartridge.

At a guess either the shower head has become clogged/calcified or the piping inside the shower has become so.

This means that water stays in the system too long, becomes over heated and the failsafe cut off activates.

Try cleaning or replacing the shower head.

the hot water heater may have sludge built up in it. drain a few gallons from hot water tank and see if this makes a difference. you should drain a few gallons fom tank at least 2 times a year, to keep the sludge from building up. ( is the tank big enough for the job)

check the water supply to and from the hot water heater

Hello, first if any water supply hot or cold is turned off, you will experience trickling as you mention. You must have both cold and hot in order to adjust the temperature on your valve. if you had a plumber intall this unit you should ask him to adjust it. Otherwise here is how:
Rotate the handle counterclockwise to stop and let water run till cold water is purged, check
temperature. Rotate handle to off, remove handle parts (see Disassembly in your manual) and temperature limit stop, rotate clockwise to decrease and counterclockwise to increase maximum hot water temperature, reinstall temperature limit stop and handle (TIP: Do not screw handle in until desired maximum temperature is achieved) Repeat steps until desired maximum
temperature is achieved

yes, the valve should be close at the downward postion. and the water should be clod to hot from left to right.
open the handle and pull out the cartridge.
make sure the valve body is installed right side up.
if it is, insert the cartridge with the lock pin on top.
reinstall the handle.