2000 Pontiac Grand Am car overheating

I have a 2000 Pontiac Grandam that I have had since March. It started running hot after I got it well about 2 weeks later; so my fiance did some investigating and cahnged the thermostat when he got to it the Thermostat housing was very corroded and covered in what looked to be silicone well he changed the housing to. The car ran ok then started running low on coolant the low coolant light kept coming on. I also heard a gurgling sound under the hood when I turned the car off. I no longer hear that but it is still losing coolant I believe. Today I started the car and an the Low coolant light came on and remined on so I looked in the resevior and it was completly empty, I thought it leaved out I added more and started the car again and it all overflowed out, so where was the coolant to begin with? Also the temp gauge will move past the halfway mark and then in about 5 minutes go all back down below the half mark. Could I have a bad water pump. I have never let the car get hot enough to crack the head but I pray that it does not have one. I know that the car always cools itsrlf down before it really overheats so what can be the problem? Thanks so much