You said you pulled the pump off the engine and it looked OK....
Did it have a metal or plastic impeller??
Plastic ones work loose from the shaft and don't spin efficiently and don't move much coolant around! They look good to the eye, and even with a spin by finger, but once load/pressure is applied, the impeller slips on the shaft.
Also check the radiator....there should be an internal baffle that directs the flow through the rad.
They have been known to be missing on cheap rads or break off on old rads.
Take rad out, invert, place a marble in one inlet and see if it rolls to the other side....if it does, rad naff, if not, time to look else where.