2002 GTX overheats at high RPM only

[tigerfit]

Looking for some diagnostic help. I have a 2002 GTX that has been in storage in a warehouse for about 6 years. After cleaning out the fuel tank & injectors, replacing the fuel pump & strainers, and doing all regular service (plugs/oil & filter/coolant change), I took it to the river today.
Runs perfect as long as I keep it under about 6000rpm. In fact I ran it for over 30 minutes at low speed. But anytime I open it up, within a minute or two the overheat alarm is going off. Not a false reading either, it's getting hot.
Taking into consideration the long storage time, where should I look? Thermostat? Water pump? Any tests I can do to pinpoint the problem, before I start throwing parts at it? I plan to take it back tomorrow with an IR temp tool.

 

[tigerfit]

Just to add...I get no other text, lights, or error messages on the dash when the overheat alarm is going off. I'm not sure if it's the engine or the manifolds, which is why I'm bringing the IR temp tomorrow. Does the lack of any other message indicate it's the manifolds? Or possibly the computer not working right? When I first launched, only the RPM was working. After a while, the fuel gauge kicked in, then later the speedometer worked after running a half hour or so. I thought it might be corrosion.
This is a twin to the ski I've owned for years, only reason I agreed to help. Mine never had this problem, but it also has had the melted dash for over 12 years, so I'm not familiar with the computer or error messages.