It's probably the gage. The gages on these cars were notoriously inaccurate. Get a lazer thermometer ($20 or so from Home Depot) and check the heat from multiple locations. Mine will hold steady at 220 on the gage in Phoenix in 114 degree heat.....but the actual water temp is closer to 190-195 degrees. Mine's not a turbo, its the 6.6L 403, but as long as the original radiator shroud is in place, the radiator is not plugged, and the water pump is good it should not overheat. I would change all fluids and put what ever oil it calls for in it.