Return of "cut out when hot"

[CROUCHY]

After several years without the "cut out when hot" issue, my Chimaera is now experiencing bigger than normal rev changes followed by occasional cut out (when it is hot, last time during MOT)!
The last time this happened I cleaned the stepper motor and got into the habit of leaving the petrol cap slightly loose.
Is there some other simple check I can make once I have cleaned the stepper again?
Thanks,
Crouchy (19 yrs of Chimaera joy...)

[Discopotatoes]

Check your dizzy cap for cracks