My son has a Ranger 4

My son has a ranger 4.0 that we had a back yard mechanic swap out engines, he picked it up from him when it was done and noticed that the temp gauge will go to normal temp then to the cool side then back to around normal but never gets overheated, it only seems to do that when driving. Then if we stop to make sure the radiator and the overflow are full we noticed the top radiator hose is either soft or slightly sucked together, so it seems like it wont build up pressure as soon as I take the radiator cap off I can hear a little air, the hose will open up and it seems like the coolant is going to the overflow tank, until it cools off then back to normal level, I thought it could be the thermostat, so I replaced it and the radiator cap but it is still doing the same thing, now I am hoping that its air in the system and not a head or intake problem, also the clutch fan always runs I thought they are supposed to free wheel, even though it will spin when the engine isn't running, so maybe that's the problem because it never gets overheated. I am confused because it does put good heat out of the vents, but glad its not getting over heated because this is a new rebuilt engine about 30,000 miles ago out of a rolled ranger. His last engine overheated and pretty much wrecked it, just trying to avoid that, let me know what your thoughts are, thank you.