Valiant turbomax plus 828e?

I keep losing the domestic hot water when the central heating is on. I can only get it back by turning off the c/heating for a few minutes, it then returns I can then put the c/heating on and everything is fine for a few days and then I go through the same process again.
I've fitted a new water pump and confirmed that the three way valve is clear and free from obstructions.
I've had the c/ heating system power flushed and added a magnaclean filter but still have the same issue.
Can someone advise on what to do next.