Plug issue?

[Dean Southall]

I treated my D1/D14 to a new spark plug and this fixed her performance problem but the other night on the way to a Club meeting she started intermittently missing. The problem disappeared after a couple of miles. I had exactly the same symptoms on the trip home.
I was wondering if a hotter plug might be indicated? Current plug is an NGK B7ES.
Perhaps she developed a need for a bit of choke until warmed up. Although has not previously.