[n4mwd]

The bike tends to run hot - especially when riding slow. But if you have coolant and flow, then the cooling system isn't the problem. What does the gauge say when you are at speed >60mph?

Is there any chance someone put the wrong jet in the carbs causing it to run too lean? When its acting up, try a small amount of choke to see if it cools it down any.

Lastly, its also possible that the gauge is simply wrong. I've never heard of this on a Ninja, but temp sensors do fail.