Hi Thanks for getting back to me.
I am absolutely no expert. My symptoms were similar and it happened whilst i was in traffic coming up to a roundabout. It felt like"chainslip" under load. I nursed it home OK keeping the throttle to about 5% max. Even in top if you applied throttle I felt a clunk in the back axle area (NB I changed the oil on my return and no metal bits in it) and this "chain slip" feeling. I initially feared that (impossibly) the back wheel was coming unmeshed with the drive in the axle. Speaking to a local afficionado he thought it was the gearbox back bearing allowing movement at the start of the prop shaft which allows it to unmesh with the gearbox final drive although Triumph dealer reckons it was only a 4th gear problem (which it was definitely not) after a 2 minute test drive.
I have to admit I do ride the bike hard but it has a rev limiter which is surely there to ration the power to the limits of the bikes transmission and the adverts say that the drive train has been beefed up to take the extra torque.
I am having the bike placed on a rolling road with (not the dealer) a trusted race mechanic trying to get to the root of the problem. Will keep you posted.