Triumph 675 "shutter" while down shifting, Help please....

While at Road Atlanta this past weekend my Triumph 675 started doing something a bit unusual and I need some help diagnosing it.

As I crest the hill from the back strech in sixth I start to downshift to second for turn 10a, I am getting a massive "shutter" or "vibration" through the handlebars, this also happens going into turn one when downshifting from fifth to third for turn one. It only happens during downshifting from high RPMs. I just removed the cluch cover and see nothing wrong. Bike has a STM slipper clutch and I have never worked on one before.

Anybody can "help a brother out" with a idea here? Do the springs wear out in these things? Bike has low milage but I do just downshift and dump the cluch lever out dirt bike style.

We swapped both wheels just to make sure it was not wheel related but it does seem to be coming from the clutch or engine somewhere. Any ideas? Thanks.