Dumb question but have you set your suspension up properly? Rumblefish is dual suspension correct? Make sure the rebound on the rear isn’t set faster than the fork. Maybe play with the adjustments a little bit at a time. Just a thought. When I first got a long travel trail bike a buddy educated me on setting up the suspension (I had the sag right but that was about it). It made a big difference. I was rebounding way too fast on the rear and I always felt like I was going to go over the bars on a rhythm section we used to ride.
If you do work on it yourself I highly recommend making small adjustments to one setting at a time. Don’t mix changes to both.