PJNH

I have a rapid chip upgrade plus have installed a larger intercooler and have fitted 2 extra transmission coolers but are having overheating issues towing a 3.4t van. Issue happens when going up inclines in 4th or 3rd gear okay when travelling at 90 to 95 kph in 5th gear. Has anybody has this happen ?experts are unable to definitive advice as to cause, am thinking to much power into transmission.