1992 Mercury Sable Tempreture changes

Check heater core hoses if both are hot ,
If both hoses hot and not enough heat inside suspect blend door actuator
If one hose hot and the other warm or cold the heater core need to be flushed (very common)
as a 1st step these are the steps
WARNING: NEVER REMOVE THE PRESSURE RELIEF CAP UNDER ANY CONDITIONS WHILE THE ENGINE IS OPERATING. FAILURE TO FOLLOW THESE INSTRUCTIONS COULD RESULT IN DAMAGE TO THE COOLING SYSTEM OR ENGINE AND/OR PERSONAL INJURY. TO AVOID HAVING SCALDING HOT COOLANT OR STEAM BLOW OUT OF THE COOLING SYSTEM OR DEGAS BOTTLE, USE EXTREME CARE WHEN REMOVING THE PRESSURE RELIEF CAP FROM A HOT COOLING SYSTEM OR DEGAS BOTTLE. WAIT UNTIL THE ENGINE HAS COOLED, THEN WRAP A THICK CLOTH AROUND THE PRESSURE RELIEF CAP AND TURN IT SLOWLY UNTIL PRESSURE BEGINS TO RELEASE. STEP BACK WHILE THE PRESSURE IS RELEASED FROM THE COOLING SYSTEM. WHEN CERTAIN ALL PRESSURE HAS BEEN RELEASED, PRESS DOWN ON THE PRESSURE CAP (STILL WITH A CLOTH), TURN AND REMOVE PRESSURE RELIEF CAP.
Engine cool
lower the coolant level
Locate the two hoses running to the heater core
disconnect them at the engine side leaving hoses connected to heater core throw the firewall
make sure you pinch and close the bypass hose between the two heater hoses
use garden water hose and compressed air if possible
run the water and follow by air throw one side and out the other and vice versa
do that two to three times watching what leaving the heater core you are looking for the good flow and clear water to run throw
reconnect hoses
top off coolant system