Creda Expressions power stutters when switched on

[RitaP]

Creda expressions electric shower. When power button pushed, power stutters with clunking noise. This causes the water to splutter. Improves when power setting is switched to low. No issues with any water pressure elsewhere in the house. Problem started after we'd been away for 10 days. Shower is approx 2 yrs old & fitted by qualified person. Please help.