T.D. Jack - 'Check Engine' indicator. Overheating?

I have a Y2000, manual, T.D. Jack with 130K on the clock.
Recently I towed my camper (about 1800kg loaded)up the SE freeway out of Adelaide. About 300m from the top the 'Check Engine' indicator came on. The temp gauge was a bit hight than normal but not what I would consider dangerously high. I pulled over and let the engine idle until the indicator went out and the temp gauge had dropped back to normal and then continued. I had made the climb in 3rd holding the revs to about 2800 which is a little over 60 Km/h.The vehicle seemed to do the climb quite easily with a fair amount of throttle travel left and 2nd seem far to low.
Given that the Jack is considered to be a reasonable towing vehicle and I was well within the 2500kg limit - was I using too high a gear or should I have the vehicle checked out.?
This is the second time that the 'Check Engine' indicator has come on when I have been towing the camper. The very first time I towed the camper involved a climb up Willunga Hill on a day of 42 degrees. Not only did the 'Check engine' indicator come on, the engine management system seemed to drop back to some sort of survival mode.That scared the hell out of me. I though I had blown up the engine!
Next year we plan to do a trip to Jindabyne and then up the East coast. There will be some long steep climbs in that lot - Mt Bulli comes to mind and a couple of places on the Snowy Mt highway.
So, have I been using too high a gear or should I have the vehicle checked out? Any advice gratefully received.