Shower flow problem

[Snappedash]

Hello All

I wonder if someone will be able to offer some assistance with this problem....

I have just fitted a new mixer shower with a diverter that acts as the bath tub feed too.

We have a problem whereby after the hot has been running a while in either the shower mode or the bath mode the hot flow drops and then pretty much stops. If you leave the valve open however and then turn the hot tap on in the sink you get excellent flow rate there and the flow in the shower/bath mixer returns.... Turn the tap off and the flow goes again!

I can't figure what the issue is on this... I have emailed the supplier but they have failed to get back to me!!

Does anyone have any ideas?

Thanks in advance