Q: VW Polo overheats on the highway

Hi, I hope you can assist me, as I am at my wit's end. My 1.4 tdi polo overheated on the freeway about 2 months ago. No overheating in stop-go traffic. A mechanic serviced it, replaced the water bottle, thermostat and its housing and flushed the system. A week later it overheated again on the freeway. A radiator shop then checked gas emissions, there was no reaction to test liquid. While replacing the radiator they found that the intercooler housing was mounted incorrectly, damaging the intercooler. It was repaired and the car's performance definitely improved. All was well for about 3 weeks, no overheating or even temperature spikes at all. On 24 December I drove further than I usually do, and while raining the car overheated again. I drove very slowly to the nearest garage, as stopping next to the road in my country is not safe and with the rain I would not have been able to see much. Someone helped to brigde the fan so that I can reach my destination, but the car kept on overheating. I stopped and waited for it to cool quite often. When returning home on 26 December, if I kept it below 100km per hour it was ok, but on hills the temp climbed. I am not sure what could still be wrong, and everyone I spoke to were confused as the car has not overheated once in stop-go traffic, only on the freeway. Any advice or ideas of what else to check/test would be greatly appreciated.

My car has 133000 miles.
My car has a manual transmission.