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I have a 97 honda civic EX coupe with the VTEC engine and a automatic trans (with A/C).  I need to replace the termostat but I've looked around and found a few different ones with different heat specs.  One is a low-temp 170 degree unit and I've seen others that are 192-195 degree units.

Which unit should I buy?  Will it hurt anything if I buy the higher range thermostat as opposed to the "normal" 170 degree OEM replacement unit?  I don't race or anything like that, and the car is not mechanically modified in any way (everything is stock).

Also, someone told me on these civics that the torque converter locks up when the engine reaches a certain temp and driving speed.  Lately, I've noticed that if I have the heater/defroster on, the torque converter won't lock-up.  But when I move the temperature to a cooler setting (cold), it will lock-up and stay locked up as it should.  I also read on Edmund's webiste that there have been complains of this (and this is part of the reason I'm replacing the thermostat) and that replacing the thermostat will fix this issue.  Any ideas or suggestions???  Anyone else have this problem of the converter not staying locked up?  The car is almost 11 years old and has 140k on it and is in good running condition.  

I would stay with the 170 , that the manufacturer suggests, as far as the lock up there is what is called a TOT sensor(transmission oil temp sensor) in the trans, and the fluid gets cooled by the cooler in the rad. if the fluid is to hot the TOT trips and can cause more problems than just no lock up.  

For what its worth ,you can buy an additional transmission cooler. It looks like a little  radiator that is zip tied to the front of the cars radiator and two rubber hoses are "tied" in to the regular transmission cooling lines. A lot of people use them. Especially in hot climates,,or mountainous areas or when the vehicle tows any kind of trailer.
    There are plenty of websites that will explain it.