Did you change the coil packs as well and did you use oem quality? Gaps on genuine plugs came preset on the ones I just changed on my 3.0 Sport, perhaps you had a dodgy fit or connection somewhere?
This happened to me about 45 years ago on a A40. Just because plugs are new doesn't mean they are perfect. I still have a feeler gauge somewhere! Peter.
Hi Andy --You say its on the easy bank so if it was you I would put the old plugs back in and if it runs OK again it would mean that one of your new plugs is breaking down under load [ It does happen at times ??? ]--you can then change them one at a time to find the bad one and get it exchanged. All the best. Frank