Check for broken motor mount bolts, also not sure how many miles, but check reeds also. mine was doing the same thing, and it ended up being a combination of both.
regardless, if it is your issue, i would check your reeds for wear. Relatively cheep, but important. Also, could be dirty carbs, broken/wore out spring in your clutches. I have also had my silencer plug up on my old 600 SX.