Hi, i have a similar problem, my TZR seems to stutter and strugle at and between 4500rpm and 5500rpm. i fingd this normally happens after starting the bike up first thing each day. when this occurs just put the choke on between this rev range or leave the choke on until it has had surficient time to worm up. the only advice i can give on your problem is to make sure your throttle cable is tight and that your chain is well lubed as this was givin my bike a similar problem...hope this helps