Overheating on start

Hey, I'm a pizza delivery driver and I drive a 2005 LS. Recently I've noticed at work that when I come back from a delivery and I turn off my car to run inside and grab another one, when I start my car after going back out it'll be slightly overheated for a little bit. It always cools back down after about 30 seconds to a minute and turning on heater helps it cool down more quickly. After that I won't have any problems on the road and it won't overheat again until I repeat the process described earlier. Even then though it doesn't happen every time, maybe about 60-70 percent of time. Any ideas?