1995 Olds Achieva heater issues!

Car Info:

1995 Oldsmobile Achieva model S 4-door
3.1L V-6
139,500 miles

About a month ago I had a "Low Coolant Level" light and so I filled my reservoir the next morning before driving. A couple of days later my car was running normal engine temps but then suddenly climbed to 250ish. Slightly in the red zone. Oil pressure reading was perfectly fine, so I pulled over thinking I might've blown a hose or something. Pop the hood, nothing is blown or even leaking a whole lot. As I pull back onto the road (about 5 minutes later) engine temp is straight up and down in the NORMAL range. A few miles later the needle climbs to RED and then back down to normal. During the swing in temp I was going constant 60mph.

I checked my Haye's car repair manual and it just mentioned the reservior. So I checked it again and I still had all the fluid that I put in before. My dad mentioned that I needed to fill up my radiator, so I checkd the radiator and the manual and nowhere did I see any cap on the radiator itself. So either I'm blind or I don't have a radiator cap.

So I keep driving and it's usually normal. But every now and then the needle will swing hot and back to normal a few times. Oil pressure is steady throughout the temp changing.

During a colder winter day, I had my car warm up for 20 minutes. When I went back in to drive home, I had COOOOLD air blowing in even though I set the controls to WINDOW DEFROST MAX HEAT. I turned control knob to max cold then back to max heat and still cold air. Engine temp is straight up and down oil pressure good.

As I accelerate, air is now HOT, temp and pressure fine. As I brake, air is COLD. Engine temp and pressure fine.

What is going on! Heater core blockage? Radiator blockage? Bad water pump? Bad temp sensor? The repair options are endless! Which one!

Thanks!