combi hot taps reducing flow when hot

[slogo]

ive had a new Viessmann Vitodens 050-W 30kW combi fitted. The property is old and old separate hot/cold taps in bathroom and Kitchen. When hot water is drawn at a medium flow and temperature stabilizes at max temp ( about 30seconds) the flow reduces by 2/3 .. I can turn on the tap some more to increase the flow again to where it was originally. The flow rate seems to stabilize after this.
This behaviour occurs more noticeably in the kitchen during washing up , but also in the bathroom basin tap.
Boiler company are scratching their heads. They think its the tap itself but logic says its isnt, as far im concerned

Many thanks

 

[cross thread]

Normally the tap,

 

[Madrab]

+1, it's the tap.

Presume it's a screw down compression type tap? As it heats up the tap/jumper/washer expands and closes the flow down a bit, that's why when re-adjusted is settles down. I have 2 new bathroom taps that do the same.

 

[slogo]

Many thanks @Madrab @cross thread
The tap it is then - now it makes perfect sense
much appreciated.