overheating problem

[W. Hsu]

hello all,
i just bought a 93 se-r two weeks ago and the motor is a JDM.  i noticed a
problem.  when i drive it hard like shifting between 5000 to 6000, the
first half hour to 1 hour, the temp is fine.  but when the driving lasts
for 1 to 2 hours, the temp meter starts to rise a bit and if i keep
driving even if i shift at 3000 or 4000, the temp is getting hotter.  this
sometimes happens when i hit the freeway.  i
stop the car immediately when the temp is half way between the normal
operating temp and the "H" zone.  i'm thinking that my thermostate is not
sensing the water temp properly.  what do you guys think?  flush the
radiator?  or get a NX radiator?  get another temp gauge?  help.