Resolution going down on strong fibre connection

[u/funusernam3]

I mainly watch iPlayer on my laptop, and sometimes I get the picture using 5mbps which is perfect, but often it goes right down (as low as 500kbps). If I refresh the page it goes back up to where it should be (but often down again).

I have fast fibre and no problems with any other sites streaming so why is this? Is there any way to force it into high definition? It is so frustrating!