1994 Jeep CJ5 94 overheats

Hi I have a 94 jeep grand cherokee laredo. It sat for about 3 months (sept-dec) and when I got it back on the road it was running fine. A week later when I went in to get a radiator flush oil change etc for a tune up it overheated very quickly. It was about 15 degrees out. Also the heat never worked unless you had it switched to heating air that was circulated from the inside of the car. As soon as I started using that, this is when this happened a few days later. I suspected there was a problem with the heat valve originally, but upon overheating I blamed the thermostat. All the hoses were brand new. When it cooled down I restarted it and felt that it seemed that nothing was going through the top hose. So I changed the lower hose for the hell of it, resealed the gasket and changed the thermostat. I never started the car with an open thermostat, although I should have. (No garage 20 degree weather, not what I wanna be out in with a small child to care for inside) Anywho, it still over heats. I changed the oil topped all fluids and drained the radiator and refilled. My friend who is a mechanic was there to oversee anything I missed. It still overheated. It goes up half way and then pins down. I started it again when the weather was nicer in the 50's and it didn't over heat as quickly. I didn't drive it off because my son was with me. It was definetly runnin considerably longer without it over heating. Anyways, my friend suggested a water pump, but I think it might be a clogged radiator? He didn't say anything about my head gasket but how can I check the water pump? My dad said something about taking a hose off and seeing how much output I get. And if it is the head gasket, how do I know? And how much is that part? I'd much rather just change the radiator haha i've done that many times before. Thanks for dealing with the rambling, but cars are like human bodies for me, all the symptoms must be discussed before the diagnosis. Although it is just nuts and bolts.