2000 Oldsmobile Intrigue Engine over heating

I have 2000 olds intrigue with 141,000 miles. My first problem is I am loosing coolant. I dont not have any coolant puddles or see any coolant leaking from any where. But my low coolant light on my dash comes on. I just add coolant when I see it come on. This has been happening for over a year. Where is the coolant leaking from. Next, this summer, I live in charlotte, nc average summer temp it 90. While driving on the highway with ac on, the car runs hotter then normal. On a few occasions the temp gage would get to 3/4. Well I turned the ac off and turned on the heat to cool engine down. That worked. So I didnt use the ac any more. From then on temp stayed at normal degree. Well, now it want to run hot all the time. I was on the highway with no ac running and the temp gage shot up to the red very quickly. I got off the highway immediately. As soon as I got off I watch the temp gage go right back down to half way, while this is still running warm it ran fine from there. I had to park the car so I could do other work on it, replaced crank shaft sensor and oil pressure sensor. After getting that work done, I took it for a short drive, around 10 min. During the test drive, it was running normal at 35 mph. Well I got up to 45 and I just watched the temp gage keep rising to the red. So I turned on the heater to see if I could bring back down. It wasnt blowing any heat. After a couple of min, about 3, it started to blow heat and at the same time the temp gage came back down to normal operating temp. I have read some of your q&a form, and from those I have an idea it could be the thermostat and/or radiator. Im I correct? Also why am I loosing coolant again?