Kawasaki Forums - Ultra 150 Overheat problem

I purchased a 99 ultra 150 at the end of last season with 122 hours on it. I had it out for the first time today and had a blast except it overheats if pushed hard (6000+RPM) for anything more than 10-15 seconds. You can gun and let off with no problem but hold it on smooth water and the warning overheat light comes on and she stumbles. It has good flow out the water outlet???? Runs A+ other wise???? Stock engine/shredmaster plate and scoop grate. I was out for over an hour before the problem appeared and stayed out over an hour more and just kept the RPMS's down. Any ideas on where to start? I was going to pull the plugs as I'm told you have to use these special expensive plugs. Other ideas would be appreciated!