If it is an older original radiator, chances are it is clogged with rust particles and water pump grease. The radiator cannot be simply flushed out. The old grease and rust is just too stuck in there. Think of it this way. You have many parellel paths between the top tank and the bottom tank. You may be successful flushing a few of these paths but then the water will just flow through the open ones and bypass the restrictions. You cannot get enough pressure drop across the core to get the velocity high enough to push through the clogged tubes.
You have to remove the top tank. I filled the core to just below the top tank to protect the core. Then using a propane torch, air compressor and nozzle, heat and blow out the solder around the seams. Remove the top tank. Then use a brass welding rod with the tip bent in a small "L" and a can of "GUNK" degreaser and spray into each tube and push the accumulated grease/rust down into the bottom tank. If you find some resistance, stop and spray more GUNK and gently start again. Be patient. You will end up with long lines of grease in the bottom tank that can be easily flushed out.
Put the top tank back on with flux and solder. My car ran at 155 F after doing this. Good luck, ED