Hopefully i can help..
What model of peugeot do you have exactly engine size etc?
Does the main fan kick in when this overheating occurs?
Peugeot 206 cc 1.6 petrol and yes the fan does kick in
Hi..there sorry for the delay..
I note you do say you have had the system pressure tested which is good as long as it has come back as normal with no issues - in that case it more than likely going to be electrical in regards to possible issues with the fan relays/resisitor block or wiring to it..firstly anyway check out the relays which are located on the fan shroud or the left behind a long plastic panel held with the 2 screws..you'll need to shift the front grille - you'll notice the wiring harness coming in underneath then up behind the panel the relays are behind this 2 of them/resistor is right at the bottom..double check these havent suffered with corrosion..the next port of call would be the temp sender/switch.could be giving out false reading bringing the temp guage up and allowing the fan to switch..but as above maybe check out those relays..I do hope this helps