I just had a very similar problem. I just put all my 22re stuff onto a good 22r I got from a junk yard, put a new waterpump and thermostat in, and changed all the hoses. I used my old radiator because it was still working well when i pulled my old engine. I drove my truck around town for a day or two, and it ran fine. The first time I went on the highway it over heated, I used a radiator flush, and ran it for a bit, drained the system, and when i did I saw a bunch of rust and scale come out of the engine block's coolant hole, but not out of the bottom of the radiator. I took out my radiator, and shook it a bit, and heard a bunch of stuff shaking around in there. I tried to flush out the radiator by itself, by filling it half way up shaking it up and draining it. I got a lot of stuff out, but couldnt get it all out, and ended up breaking off one of the side mounts in the process. So I got a new radiator, put it in, and it has been running cool at any speed on the highway ever since.