Hi, I have a Y61 ZD30 with about 440,000 on the dial. Recently while towing the camper trailer, not light, had an overheating when climbing through Cunninghams gap. Dropped it back a cog as usual and kept it running around 3000revs as normal but the temp guage went right up. Very unusual because it always runs mid range no matter what. Checked the water and all OK. Any suggestions.