Autobox overheating

Hi guys. I have a 300tdi defender with disco autobox. It all runs sweet until I attempt to tow any significant weight.
It runs a 1:4 transfer box and slightly bigger tyres than stock. It seems the box starts to generate enormous amounts of heat which in turn causes the engine to over heat. I can only assume that the build up of heat is to much.
It does have a td5 intercooler mounted in front of the rad. Would this reduce to much air flow through rad? Normally when there is no load the temp sits happily below the half on guage.
The autobox has a oil cooler but it is mounted in a stupid place under drivers door on side of chassis with no air flow.. Do you think that increasing the size of cooler and mounting at the front would keep everything cooler. It has to tow and if it can't sadly it would have to go. Any inputs will be appreciated.

If all rads in good order check engine and oil cooler thermostats are working.
Uprated coolers available from Allisport, Fit up front where it gets an air flow.

I have always used a 19 row cooler on the auto boxes on two Range Rover Classics over the years under advice from Ashcrofts.
These were mounted vertically, in front of the main rad. I've never had a transmission oil temp issue.
If your engine is getting hot while towing it's not the fault of the gearbox. Either your rad is partially blocked or your cooling fan is being under driven, but please put your gearbox cooler out front in the airflow. Heat is the killer of the 4HP22.

Thanks..yes I definitely intend to move the cooler. When you say the rad is partially blocked, do you mean internally. It does have the intercooler sitting in front covering half of it.. The coolant system is very clean. Obviously can't see inside rad but it does look clean from what's visible. It's always the gearbox that gets hotter and hotter especially on hills then that's when the engine overheats