TR250 Overheating

[Highway Star <highwaystar@juno.com>]

Ok great people:

I've got an overheating problem. Guage ok for first fifteen minutes, in
the middle.  Creeps upto the red mark...in idle/street driving...highway
driving drops it down to 3/4....1/4 away from the red mark!

1-Airflow is good, 
2-shroud is in place (have oil cooler that masks 1/3 of the radiator!)
3-Changed coolant

Water pump? or radiator clog?

Thanks-
Pouya