1986 Chevy Celebrity Coolant acts weird after turning off

Engine Cooling problem
1986 Chevy Celebrity 4 cyl Two Wheel Drive Automatic

Hey there!
I bought a used 1986 chevrolet celebrity, 5 doors. It just got 60.000 miles - its been sitting most of his life in the garage. I already drove this nice vehicle about 400miles, but two times something strange occured:
the temperature showed 260F, and after I shut off the engine the coolant went back to the coolant container with a VERY loud noise, I think that air came out of the system as well. It rushed and nearly overfills the container. When I start the engine again it just sucks the coolant into the radiator and a normal level of coolant is present in the container. It happened a second time today when the machine idled for about 40 minutes (I refilled my AC cooling fluid). When I shut the machine off it made this extremely loud noise again. I noticed too that minimal amounts of coolant (engine) escaped out of the radiator cap - its not the original cap, but a fitting replacement from Stant (I will check the cap tomorrow again) Has anyone of you guys got an idea what could be wrong? I'm not sure, but I thought that it could just be because of the leaking radiator cap/fast dropping pressure level. I checked the cap when I bought the car and thats just 400 miles ago. So I hope I wont have to replace the whole radiator because the cap leaks.



Thanks for your time in advance!