Overheat at 70 mph, then instant normal temp
 

2000 Grand Marquis, new electric fan, radiator, thermostat and water pump & temp probe. Works fine in town with A/C on, on the highway at 70 MPH thermostat will flash at full heat deflection, then drop to normal temp. car will falter or limp mode while gauge and engine light flash and alarm. Pull over and let idle for a few minutes and operates fine as long as I stay below highway speeds? Still blows heat while showing extra hot.

Overheating under those conditions usually is a waterflow issue.You could confirm the temperature with a scan tool under those driving conditions. What I would look at is the lower hose colapsing, limiting the flow of coolant at those speeds. I would repace it if it doesn't contain a coil spring inside.

Another first timer looking for easy solutions. Post, run then hide.Why do you waste everyone's time?

Replaced upper and lower radiator hose last night, didn't get a chance to get it on the highway to see if the problem was solved yet!

Thanks for the update.

perform full radiator flush with Prestone followed by distilled water flush, then fill with new coolant.
If still clogged, change out the t-stat

New radiator, fan, water pump, thermostat, ECT probe, upper and lower hoses, and intake manifold! Heat must be backing up from a partially plugged cat, or CHT probe?

If you have a scan tool, I would connect it, drive it at higher speeds and see what temperature the computer is actually seeing from the sensor. If you had a blocked converter, I doubt you would be able to obtain 70 mph. A vacuum gauge can verify any appreciable restriction in the exhaust system.Some engines have two coolant temperature sensors, one for gauge and the other for the computer. If your radiator has calcified deposits blocking the flow of water through the water tubes, I doubt any cleaner in a can is going to clean it.
You have to determine if the engine is producing too much heat, false information as determined by the computer or a cooling system not performing.