Video problems

Hi,
Zoom has worked perfectly for well over a year but now there is a problem that I cannot resolve.

Zoom meetings work fine until a third (or more) person joins the meeting. At that point my video (I am the host) starts ‘jumping up and down’ (jerking). Everyone on the call see this instead of a steady image from me.

Having checked all settings that I can think of and even reinstalling Zoom, I cannot find an acceptable solution.

The work around solutions are to switch from using pc and use a tablet instead or (wash my mouth out) to close the zoom meeting and use MS Teams instead.
There are no problems in MS Teams. That might be a clue?

Can you suggest any solutions?

Geoff