1994 Jeep Cherokee thermostat

i just replaced the thermostat in my 1994 jeep Cherokee sport 4.0 motor cause i noticed the temp gauge was getting in the red my problem i am having now is the temp gauge still going in to red at city speeds but not at highway speeds i turn on my heater to draw heat from the motor at highway speeds the heat comes out hot once i get below 55 mph it blows cold and the temp gauge goes in to the red what else could be the problem i should also mention even though it is registered as a 1994 all parts i have had to buy for it are for 1993 or 1992