Dewhot LP gas geyser temp fluctuations

I had a 16 liter constant temp LP gas geyser installed in Oct 2023. The first 2.5 months it used 19 kg gas but since then the usage increased to 19kg gas a month. Constant temperature fluctuations, hot to cold and hot again, was experienced often with the temp increased to 60 degrees and then suddenly went cold and back to warm again (temp set at 46 deg). After a year the installer eventualy replaced the temp controller unit . After that the gas usage was less but temp fluctuations still occured occasionally. Today the temp increased to 80 degrees, later dropped to 70 untill the water tap was closed. When the tap was opened again the temp increased to 47 and remained there (normal).