The issue you're experiencing is likely due to a problem with the thermostatic cartridge within the valve. This component regulates the water temperature, and if it malfunctions, it can cause inconsistencies or problems with temperature control. To resolve this, you'll need to either clean or replace the cartridge. Cleaning may be sufficient if the issue is caused by mineral buildup or debris. However, if the cartridge is damaged or worn out, replacement will be necessary.