2008 gsxr 600

I just bought a 2008 Gsxr 600 and when I test drove it, it ran like a champ. But after I got it home and did some cruising around the city I slowly started to realize that between 4-6k rpms constant speed (no matter what gear -but mainly 3rd, 4th, 5th, and 6th) that my rpms would do it to 0 and...