Hi Steve, I have had a number of vehicles that have had symptoms just like this in the past, some struggle to run others run fine, it only seems to take moments for the exhaust manifold to become cherry red. In all cases it was down to a misfire situation and excess fuel entering into the exhaust and overheating the system, a set of ign coils has cured every one.

I have never forgotten a Renault scenic; I drove 1.5 miles to the dealer to pick up a set of coils and on the way back I wondered what all the smoke and hot smell was coming from the car, it was dark and as I drove into the yard one of my mechanics wondered what the strange red glow was under the vehicle. The whole exhaust from manifold to tail pipe was about to melt and the rubber mounts were smoking and just about to catch fire, we never got the burning smell out of that car. I have even had them where the misfire has been virtually undetectable and the exhaust heats up excessively with no misfire codes recorded but coils have cured the problem. I would guess the excess fuel flashes off the CAT if it is an efficient catalyst and raises the temp of the exhaust. Hope this helps.
John Kennard.