Erratic temp Gauge 05' Buick Rendezvous

Thank you for your quick reply. I’ve owned the car for 3 yrs and have not experienced this dramatic fluctuation in temps before. The car has always remained cool even while stuck in traffic for long periods. I live in the south and have traveled up and down the coast subjecting the engine to long drives and extreme temperatures. It’s always been great but one day last summer while waiting in traffic I noticed the car was near overheating. I quickly turned the heat on full blast and was able to start moving. Now the temp gauge shoots up and down whenever I have to let the engine idle hot for more than a few minutes. I don’t dare risk NOT turning the heater on (as a precautionary measure)when it occurs for fear of getting stranded somewhere far from home. I work out of the country, and will surely take it back to the dealer when I am Stateside next month. Just wondered if anyone had experienced the same behavior.