Problem 1: Bleed the system. You have air in the system leading to temperature gauge going up and down. Air pockets lead to heat up and uneven temperature ranges.
Park the car on slope with front of car facing up. Open radiator cap. Start the car and let it idle for 20 mins with heater ON. Keep on adding coolant. Once it starts to overflow close cap.
Problem 2: A minor leak in the system. Check heater core, hoses, water pump, radiator.
If hoses are old and hard replace with genuine part.
Best diagnosis? Go to workshop that does pressure test on cooling system and see for loss of pressure.