Offroad-Express

I have a 97 Mistral the one with the round headlights/bonnet scoop. Anyhow it has an intermittent fault, this has occurred twice in the last three weeks. Both times it has occurred on an uphill section of tarseal doing about 100kph it was raining steadily both times. Both times I was towing a light trailer with two dirt bikes aboard (all up weight 400kg I would guess). Basically it is as if the truck was misfiring, the closest thing I have experienced to this is water in the fuel of a petrol powered vehicle. The vehicle loses power and kind of bunny hops. Both times this occurred for about a minute or so and then went away. I checked and the fuel filter has done 25,000K so probably due for a change. The truck had a fully rebuilt diesel pump fitted 25,000K ago and generally it goes great apart from this weird intermittent fault. Can anyone tell me what the issue is??

I would say you are getting some sort of fuel starvation...
The fact you are going uphill and also pulling a load which will cause you to want more fuel...

The weird thing about it is it's very intermittent I drove it from central otago to Dunedin today so lots of hills and it only did it on one hill. On Tuesday I towed a 4M alloy pontoon boat plus 3 guys and gear into central otago and no issues but it was dry. Today it was pissing down rain and it was pissing down the only other time it has done it too. Might just be coincidence but.....

Thought of possible air leaks around air intake and rain getting blown in and dampening the charge?
As its only happening when liaded and wet inplies that the throttle is open allowing for more flow. When dry all good leak fine.
May also be a case that as water is being pushed through the rad the fabs are then spraying moisture everywhere possibly soaking the either a electrical connector or again an air intake junction