My car is overcooling

I have a 93 civic cx hatchback and now that cold weather has arrived i've noticed a strange problem. When the engine is at normal temperature at idle and i turn the heater on the temp will drop almost down to cold in about 5 minutes is this normal???. I've checked the themostat it opens and closes fine and the gaskets and seals are fine, should I put a higher temp themostat in or is there some kind of cooling system problem.

I dunno drop in voltage to the temp sensor? get out there with a voltmeter and check things over.. I do notice in my car there is a definite voltage drop wfrom when the car is in driving revs to when it is sitting at a light with the heat on high.. you notice the lights dim a bit from normal driving, and the fan actually sounds slower (when at full blast) until you start up again, and everything goes back to sounding like normal, and the lights brighten up.

that is normal, the heater core takes the ambient fluid from the motor and pushes it towards the core, cooling the motor.

Get a higher temperature thermostat, nothing lower than 180, preferablly a higher temp than that...Like crxlvr said, the heater core is acting as an extra radiator, and that is exactly what it is! So when you use your heater it's cooling down the coolant too much and your coolant isnt warm enough - I bet your heater isn't heating as well either huh....

try replacing both the thermostat and the radiator cap, most people overlook the radiator cap but it is actually a very critical part in the cooling system because it keeps the pressure in the system which is what determines the boiling point of the coolant.