[DeTomaso] how come my Cat doesn't much care for water?

[Kerry Maguire]

I took my car to my mechanic's house a couple of weeks ago for a "look see". We checked the car out generally, everything was fine, but we found that the water level was real low in the pressurized overflow tank and non existent in the other, non pressurized, overflow tank.
 
The car runs perfectly fine and never runs warm. We topped off the tanks and I was on my way.
 
Today, I took the car out for a VERY spirited run.  The outside temp was 95+ degrees, I had the A/C running and the car's temp gauge was right on top of the "9" on 190 degrees on the temp gauge. Everything was going swimmingly.
 
However, when I pulled the car into the garage and parked it, after a couple of minutes it started burping some of the coolant from the non pressurized tank.
 
Should I let the car run for 5 minutes or so at idle to prevent this? On the other hand, when I turn off the vehicle. the temp is well within normal range. Once I turn off the car, the sucker fans shut off. 
 
Why does my car run cool but still not want to keep its recommended fill of coolant? 
 
Regards,
 
Kerry M. Maguire