98lswon1

Ok so i have recently changed the plugs,o2 sensers,fuel pump and thermostat. Before i changed the thermostat the cars tempature needle was laying all the way to the left. I put the stock 180 degrees stat in it and now when i start the car it warms up to half way and blows warm heat. When i start driving it starts cooling down instantly. It was cool all the way down to where the tempature gauge is reading nothing on the car. It has a severe lack of power also. Oh and i replaced the pcv valve and the pcv line because it had deteriorated. Anyone have any ideas on what to check? Do i still have air in the lines somewhere? i always thought when there was air in the lines that it would overheat not cool down. Thanks for any input i am working on fixing this today so anything helps that i can check.