HELP: Car Idealing at 1000-1500 RPM.

Hello:
I am hoping experts would be able to guide me with this problem. I have been
helping my friend all day today with this unusual problem. It is a 95 Honda
Accord 4 Cylinder (non VTEC and SOHC) with manual transmission. Here is the
problem:

(NOTE: Before this problem the radiator was leaking.)

After a cold start (for example today) the car heats up and the coolant
temperature needle goes its usual 1/4 on the dial. However after say 10
minutes of driving the temperature needle starts moving down slowly and if
he is stopped at a light the car idles it 1000 rpm range. After a while, the
temperature needle dips all way to the bottom and car then ideals around
1500 rpm. A healty rpm reading should be around 600-700 range.

This is what we have done so far but no result:
-Changed spark plugs and oil
-Replace thermostat
-Change the Radiator
-Coolant flush (with a bleeding tool) and fill

Has anyone experienced this before? O2 Sensor and valve adjustment is my
next guess. As usual I appreciate your help.