cooling issues

I have a 1990 lx that is mostly stock other than gt-40 intake and underdrive pulleys. the entire cooling system and fan is stock. All summer when i have been driving when the car first warms up it gets really hot, going by the stock gauge. then it would cool and run at normal temps the rest of the time. The past few days it has been getting hot and staying hot unless i am on the highway. The car has not overheated on my but the guage is up at the n and o. I replaced the thermostat with a 190 when i did the intake in the spring. Any ideas on what should look at first?

The problem is fixed. I think it was air in the system. I drove the car down the street to a friends and it was running hot. He opened the radiator cap and it overflowed since the car was hot. Then you could hear air coming out. I put some more coolant in andnow the car is runningnice and cool all the time like it used to.