I have a '97 Isuzu Rodeo with strange overheating problem?

Several Things; Check that Both Radiator Fan (Cycling as engine Heats & Cools) & A/C Condenser comes on and stays on. Make Sure Radiator/Condenser Surfaces are Clean & Fins are Straight for Max. Airflow. Your Thermostat is Sticking and Needs to be Replaced, Boil/Flush Out Cooling System and add New 50%-50% Anti-Freeze & Clean Water "Used all year Round".