you may have air in the system, on some of those cars there is a bleed screw on the top around the thermostat housing, to let all the air out of the system,i would make sure it was not that before i change the transmission ( Is this a standard or automatic?). When you drive down the road does it cool down, and them get hot when you stop. make sure the cooling fans are working as well. you may just have a bad Coolant temp switch. my wife's would run hot sitting still but when you went down the road it was cool, and it was only about 1/2 gallon low of coolant. i bleed the air out of the system and filled it no worries now.