The temperature gauge will ONLY show accurate temperature as long as there is coolant/water in your radiator. If by any sort of leak, the water or coolant have drained out then the needle will stay at COOL and suddenly jump to Red hot maximum in few minutes. I have personally witnessed this in my mehran. You need to check your entire radiator + rubber connections for any leakage. Replace all rubber tubes and attach them with good quality jubilee clips. Replace the radiator if it is too old and corroded.
Remember. You take care of your car and your car will take care of you.