declining shower pressure

[mk2phil]

My shower and bath both run off the same pumped supply from the HW cylinder using a 3 bar pump. When in the shower, after a while the hot water pressure drops and the flow starts to trickle out of the shower head. However, I've noticed that if I turn the shower off, then turn the hot water tap on the bath on, the water comes out of the bath tap slowly at first but after a few seconds it builds up to full pressure. If I then turn the bath tap off and the shower back on, full pressure seems to have been restored! This will be ok for a while then i'll have to do it again.

Any idea how to solve this and what causes it?

 

[Expertboy]

What make of shower is it?
is the cold water to the shower mains?

 

[mk2phil]

Its a hansgrohe I think.

What do you mean cold water to shower mains? The cold feed to the pump comes straight from the cold water tank/mains feed.

 

[ChrisR]

How does the cold behave, is it off the same pump, and is the bath hot tap alsp pumped or just on gravity?

 

[mk2phil]

The cold is also run off the pump and is spot on - always has decent pressure. The bath hot and cold taps are also run from this pump - there must be a tee somewhere to spilt the flow I assume.