I have a 92 civic that runs great but when the heat is on and I'm driving the temp gauge drops to cold and the heat gets cold, once I stop driving and idle for a minute or so the gauge rises back up along with the temp of the heat. Any reason to why this is happening?

 

thermostat stuck open most likely.
very easy and cheap fix...

 

^this

 

The engine may be low on coolant and/or cooling system needs to be bled.

 

Change the thermostat and then check your heater hose coming off the head from under the dizzy. The dizzy o ring goes bad causing oil to leak on the heater hose which causes it to swell and tear/blow. Might be leaking coolant from there. I have had that problem many times. On my eg and my cd5

 

I just realized. The term dizzy is for the distributor. Just to let you know