>"ps. Isn't a IRC GP1 a copy of the Kenda 270?"
Yes... just suggesting different tires rather than pulling your bike apart or adjusting suspension.
If you've mounted a 606 on the back of your bike, check it also. I mounted one on my DR350... it created an intermittent vibration like you're feeling. I pulled the back end off the ground, put the bike in gear... the tire wasn't round, even though it was mounted properly on the rim.
"I don't really know, I've been too busy falling down."