Thanks for the reply Mike.
I was aware of the situation with the FT857D, but I have now put the radio-end remoterig box on its own PSU and the situation is exactly the same. Anyway, it used to work 100% fine, so I'm wondering what has changed? I just hope it is not something being damaged slowly, that will eventually fail.
There's a possibility the power-on problem got worse after a firmware update, but that might just be coincidence. I will try an older version and see what happens.
Mark