I demoed a FSR Xc last year and had the opposite problem with my 6' 4" frame on a large,
I think the route of the problem is the angle of the seat post, it doesn't pount at the bottom bracket
therefore as you raise the seatthe top tube sort of gets longer and the seat angle slacker.(if you check the spec the seat angle is quoted as 71-74 degrees) I was miles back unable to keep the front wheel on the ground. I sort of improved thing moving the saddle around on its rails, but I really needed an in line post
My hunch would be that you just need the saddle moving back in its rails or a seat post with more lay back
I was put off at first but I'm going to try again next spring. You need a dealer who will sort all this out for you as part of the bike setup. Bike treks in Ambleside said they would sap things like saddles and stem even after you'd been for a few rides to make sure you had the correct set up. I think swapping stem tyres tec is par for the course on any bike, but should be done by the dealer at cost, eg swap a 50 pound seat post for a 60 pound one and the bike goes up by a tenner
Next year I'll demo the same bike and see if I can replicate the position of my current bike before I set off