Strange idle. Well, in the last year changed fuel pump (diff mechanics) twice. Wow, what a price tag, new radiator, water pump, serpentine and Idle pulley and fuel filter. It idles now like I pulled out a spark plug for 10 minutes at start, then idles and stays @ 1200 with Turbo gauge between White & Yellow, just hangs there (I checked for leaks today since it was 70 degrees outside). Not smoking at all, idles does not alternate, but flairs ever so slightly, so some say it's not the Turbo, and I believe it cause it drives ok (not Turbo style no more) though. What in bloody heck can it be now !? I'm gonna lose it !

Te first obvious question is the coolant level full with no air in radiator, loose serpentine belt, is the cooling fan running, especially at idle . What are signs of a bad water pump?
Five Signs Your Water Pump Is Failing

• Overheating. A dead or dying water pump cannot circulate coolant through your vehicle's engine and, as such, the engine will overheat. ...
• Coolant Leaks. Coolant leaks from the water pump are common and a clear sign that it's time to replace the pump. ...
• Corroded Water Pump. ...
• Whining Noises.

I would look at a new water pump. the impellers are stamped steel and erode.
https://itstillruns.com/install-water-pump-saturn-6674985.html

Check your radiator for cold spots when its warmed up. I think you may have some blockage either in your radiator or the hose going to your heater core. Also check the serpentine belt tension and the pulley on your water pump.

start with check fluid level --cooling fans operation --water pump to start

Hey, I see this kind of problem all the time. You should check to see if your radiator fans are coming on at idle. Because most likely they are not so that's why it over heats while it is at idle....there is no cool air cooling down the engine. Hope this helps!

Not sure this helps but I have run into several Ford pick up trucks that will idle all day but the moment it was put into gear and accelerated it would die. Fix was a new fuel pump. Had enough pressure to idle but not to take the gas. Your question did not include a make or model but first thing would be to check the fuel pressure at idle and see if it drops an acceleration. it is possible to have pressure but not volume.

Well you could have a bad water pump that is not circulating the water right while you driving you car on high speed but if you wan to try if is the thermostat take off and drive the car like that if the problem goes away just put a new thermostat

install cooler running thermostat and drill 2 hole 180 degrees apart in the thermostat. bleed system twice. make sure you get all the air pockets out of the system.

did you check the vacumm hoses ? the one on the EGR, high idle means you have a vacumm leak

Have you check the radiator fan? The're 2 of them but 1 work with the AC.

I'm sure you have installed a fuel pressure gauge and monitored the fuel pressure while the problem is occurring?Another diagnostic tool which is easy to do,is while the problem is occurring,unplug one sensor at a time beginning with the Mass Air Flow first,.if it uses one.Many times,not always,depending on the sensor,if the sensor is taken out of the picture as far as what the PCM is seeing,like unplugging the MAF,the PCM will substitute the missing sensor output with a generic value that will allow the engine to at least run,and if unplugging for example the MAF,the engine idle speed returns to normal,you know you're onto the problem.You didn't mention if the engine ran ok other than the idle problem.As far as checking for vacuum leaks,i use something like brake cleaner or WD 40,more of a liquid,and it does sound like a vacuum leak.Plastic intake manifolds do develop nearly invisible cracks that open up more with heat.Revsisit the possibility.And as for the PCM,it is possible,but more likely a mechanical problem.Lastly,remove an O2 sensor or two before the catalytic convertor,run the engine and see if anything changes.Cheap way to verify the catalytic convertor is flowing and not restricting engine airflow,as i have seen bad cats cause misfire codes and strange problems.Good luck.