New Combi boiler, shower problems

We have just put in a new heating system, changing from warm air system with miras Atom shower (plus pump) to an ideal Combi Erd 30 unit. For this to work we had to remove the pump!

When we tried it first we had cold water problems but we found out this was due to the PRV on the cold being turned down.

Now however the shower starts off as normal, good power good heat but after a min or two pressure drops and suddenly water goes cold and after a minute starts getting hot again... the pressure is still low, in fact I could pee better!!

Any ideas on what the problem might be and how best to resolve it!

Thanks