Overheating problems in 93 Mercury Sable

[Christiana]

I have a 93 Mercury Sable, I was having overheating problems, they started the day before yesterday. The temp gauge would go all the way up to the top, and it stopped blowing hot air. I tried putting in more antifreeze, but that didn't help. So I went into CarX today and the guy said it was probably the thermostat, so they replaced the thermostat. The drive home was fine, but I went to go pick my boyfriend up from work and it shot up to the top, but came back down. I was a little nervous but got there with no problems. While I was idling, waiting for him to come out, maybe 10 minutes, the vent started pouring out cold air and the temp light shot up. I turned the car off until he came out, and the rest of the ride home was fine. Is there still a problem? I asked several male friends and got responses like the radiator fan might not be coming on, and someone else said that it might be air in the cooling system. I just paid $70 to get the thermostat replaced, and the guy quoted me $300 to change the water pump.. I can't really afford to do that if its still not going to fix my problem. Any suggestions?