Are you sure the radiator is clear? had a similar situation and eventually had to remove the radiator and power wash between the fins. make sure you can seen through the radiator for most frontal area.
A good horse power rule of thumb for rotary cutters is five horse power per foot of width. Five foot would call for a minimum of twenty five horse power at the PTO. The 2740 is rated at twenty seven tractor horse power. The PTO horse power will probably be between 18-20. You may try taking a three quarter cut and see if the problem continues. You may have to make multipile passes in tall heavy material.
I just bought a 2740 but haven't used it yet but had a similar problem with a Hinomoto E324 with a 6' cutter. If the radiator is old and the fins bend easy it may not transfer the heat very well. I Had mine recored at a local radiator shop. At the very least the can flush it and let you know. May be asking some questions as I start using mine. Good luck