I have had this prob. When I forced the cpu off the H/sink, it flew off and a couple of pins were bent in the process. I straightened them carefully with a pair of pliers and with a bit of jiggling it went in ok and is working fine.
More to your problem of separation.Try using a nylon string, the type that is used on fishing rods and ran it betwen the cpu and h/s like you would cutting cheese. This was suggested to me when I posted the same prob. Good luck. BTW, I cleaned the glue which had harden from the h/s and cpu with a bit of white spirit at the end of a cotton bud. worked a treat.