This may be 2 separate problems.

The bike will happily cruise along at 70-80 mph all day.

It stutters and misfires at full throttle in the higher gears but does continue to pull - I thought this might be a breather problem so I tried it with the fuel cap open and it still has the same problem.

It randomly cuts out at low speed although I haven't found a pattern. It did it a lot when filtering through traffic a few days ago. Today it did it a couple of times at very low speed and once slowing from 50 down to 40 mph.

After cutting out I have to come to a stop and wait 10 seconds before hitting the starter again at which point it fires up straight away and I can ride away.