1998 Nissan Maxima temperature gage rises and falls

This car is a salvage title and was sitting at the buyers parkway for a while. The car runs smooth and fine. But from the second day after I bought it, the temperature gage rises and falls along with the acceleration. No oil/coolant leaks can be seen and I had thermostat changed today but it did no good. One mechanic said the engine is bad and needs to be replaced. I don't want to believe that. Please help and thank you for your time.