Over heating

My 2003 Chevy 2500 is getting hot and leaking out
of the overflow hose. I have changed thermostat
and it seemed to work for about 2 weeks. But I left
it running  the other day for about 15 min while I
was working on the inside of the truck and it got
hot and overflowed again. It only gets hot at
standard still not driving down the road which i
understand because of air flow as you are driving.
Would anyone have any advice. Of course I'm not
leaving it running anymore, but after I put water
and coolant into it I watch my gauge and it stays at
around 190 to 210 as I'm driving. But as soon as I
get to my destination  there is steam, coming from
under the hood and when it cools down I have to
add more fluid. There is no water in the oil I keep a
very close eye on that. Plz help.