Interested to find this one out...cos I have the EXACT same problem
Coulda sworn it was a carby problem...Ive had the jets,filter replaced and checked twice now...still the same issue.
Is this with a 32/26 on the A12 like mine Trav?
Seems to happen right in the middle just before the second barrel opens (im assuming)..unfortunatley at the absolute WORST range for city driving.