My truck started overheating yesterday while idling?

My truck started overheating yesterday while idling not much but it was over the line when I drove it was all good. But also sometimes after I drive the reservoir will be full and sometimes it will be empty? I'm thinking the thermostat is not opening and closing all the way. But I kind of don't think it's the thermostat because all the hoses are hot so I know there is circulation. I just installed a brand new radiator last Friday because it was cracked. Yesterday when noticed it was overheating while I had plugged in a pump to the outlet, I unplugged the tire pump and the temp started going down.