Engine temperature fluctuation?

I recently replaced a cracked cylinder head. All temperature and fan functions were normal. Now I have an abnormal temperature indication (220 degrees) at idle which returns to normal at road speeds at about 2000 RPM. A dealership coolant change used only one gallon (orange coolant) without flushing. Sometime after the coolant change, I drove in a 107 degree heat wave. The engine temperature went up to 240 degrees but never did go into the danger or "red zone". I now suspect a problem with the water pump. No coolant loss observed and the weep hole looks okay. Could the impeller be spinning loose or corroded away making it ineffective? To date I have over $3,000.00 in repair costs and I'm looking at another $500.00 if the pump is replaced. The vehicle was an absolute gem prior to this problem and I wish GM still made them!