2004 Jetta 1.8t automatic, randomly overheats, sometimes in stop and go traffic and sometimes on the highway. Checked the thermostat and it is opening correctly. Checked the water pump impeller and it hass no broken fins and does not spin freely on the impeller. Drove it thirty miles on the highway and it held normal temp for 8 miles then the temps started climbing. I turned the heat on and bled the heat off and after 2 miles it cooled right back down and was fine for the rest of the drive. This all began two weeks after I had a turbo failure and installed a new rebuilt center section. Thought it might be the gauge but you could start to smell burning when it really started to heat up and had to pull off the road and shut the car off. Two minutes later the temps were back to normal.