Problem with car heating up

I have a 94 Camaro 3.4L engine. I have replaced the thermostat twice. My car is not heating up even after the car has run for a while. It just blows cold air. In the summer sumetimes the temperature gage get's high. I don't know what to do and the automobile manuals don't offer much help. Could anyone who has experience with this or can offer a solution help, please? I'm freezing. Thanks in advance.

vacuum problem?

flush the coolant system to see abotu the summertime overheating?

heater core?

Flush the core. Make sure you do it more than once with high power. I had to do it a few times because there was rust and dirty shit coming out when I flushed mine.