Help Please

I have a problem with an early 80's 30HP Evinrude

The motor runs fine on 2/3 – ¾ throttle. It also runs sometimes well a
full throttle but usually only for 2-3 minutes, the engine develops s
misfire just like the choke has come on. This misfire continues at all
speeds including idle, the only (mostly) reliable way to clear it is
to switch off and restart although this does not always work.

I don't think it's fuel starvation as a few minutes at low throttle
/idle should clear that.

I have tried applying the choke in the hope that it had come on and
that the application would release it a few seconds later and that
this would clear the problem

Any ideas?