Faulty Thermostat?

[NSX-GT]

Hi all, there seems to be a problem with my 91 NSX... The temp guage doesn't seem to go up (always dipped down pass the blue cold area), even after warming up the car and driving normally for 15-20 minutes. During this time, the NSX doesn't put out any heat in the vents. I know the temp. guage isn't broken, because the needle DOES eventually go up after a long time.
What seems to be the problem here? Is it the thermostat? If so, should I replace with an OEM one? Or pay a little extra for an aftermarket one (mmm... MUGEN or SPOON ) I don't track my car, but I do drive "vigorously".

 

[KellyJoe]

Sounds like it's stuck/hung open to me. You probably see a temp rise sitting still and a fall while driving. As far as what to replace it with? IMO you can't go wrong w/ OEM.