Car overheating? What is the problem?

I have an older car 96 Olds. In the warmer months the temp gauge seems to go a little high. The hottest temp it goes to on my gauge is 280 and the needle gets to approx 240 at times. When the car is moving fast like freeway driving it stays lower 210-220 maybe. When I am city driving or idoling it goes right up high. Would a coolant flush and fill help this problem? Also it is ok to drive a car running hotter like this. I have to wait a week for a payday to get it looked at. Any ideas would be helpful. I just don't want to ruin anything. Thank you.