3.9 efi misfire when off roading.

I have a 3.9 efi that runs perfectly on the road, at any rev and when driven hard, but as soon as I head off road and try to climb slopes it misfires and cuts out!
I have checked the tank and pick up on the pump, I have also changed the fuel filter, but still misfiring.

Anyone got any ideas?


Thanks

Baffles in tank? Is it auto it manual. I assume your in low range?

Thanks for the reply, It's an auto and its the same in low or high, it feel like its fuel starvation, but I can hear the pump priming on startup.
Not sure what you mean by baffles in the tank? I've brimmed the fuel and it still does it.