Gwagensteve wrote: Post a photo of the radiator and engine bay. Post a photo of the front of the car.
It's possible the system wasn't bled properly when the radiator was changed. This isn't a complex problem to solve, you just have to be methodical and persistent.
Jeeps are hardly the pinnacle of reliability.
What Gwagensteve said!
Following this thread to here, I believe that the water pump - as some suggested - is on it's last legs. At idle, what remains of the corroded aluminum (sp?) blades are circulating water. Once you rev the engine and hold it at speed the blades cavitate causing nothing to circulate which causes overheating.
Because of how the front of the G-series engines were designed, it is always a good idea to change the water pump every time one changes the timing belt as the pump becomes easily accessable during that task. Bonus: water pumps are cheap! If this vehicle has not had a fresh timing belt in a while, Now would be a good time to change the belt AND replace the pump. When you do, you'll probably find out why you have these problems.
The only other problem that pops into my mind would be the fan clutch, but you indicate it is good....
Finally, these are old vehicles. They need regular maintenance - just like a Jeep(tm) does. The great thing about a Sierra and a G-series engine is that they are very easy to work on. It follows that your mechanic will probably charge you more to keep a Jeep(tm) running than your Sierra...
Try to send us pics in your posts. They are often very helpful!
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