hot and cold

Hello folks Installed in my flat is a Triton Ivory 2 8.5 KW electric shower model number H12. I am informed by my tenant that the shower occasionally will run hot, then cold, then hot. I visited and ran the shower on my hand for 5-7 minutes. Adjusted the temperature up, then down, then back...

I have a Potterton Gold Combi 28 HE on a brand new system following full renovation. However I am unable to get a consistant flow of hot water to any outlet. The temperature holds as hot for around a minute before fluctuating between luke warm and warm. Boiler has previously been sat in dry...