my rad had been done only 6 months prior to me getting mine.
However I did the timing belt and other stuff, so I flushed out the block and back flushed the rad. Some crud came out, even after the prior clean.
I also found that my aircon condensor was totally blocked, I couldnt see thru most of it when the rad was out. Grass seeds, insects, birds, hyundais etc
I back flushed it with water.
I have also gotten a piece of hollow 5/16 tube, put a small hack saw blade cut in it at the bottom where i have blanked the end off.
Fitted to a hose, it can be fitted down between the rad and condensor, and back flush the crap out of it.
When you had the rad cleaned out, you did clean out the block, and heater unit didnt you??
It sounds like either a blockage external, or internal.
Its working fine, untill you put it under extra load, then it cant cope. and is not exchanging heat correctly.
Could be worth flushing your block, and the rad again, and see what crud comes out.
Ive done rads before, and in 2 months, they needed cleaning out again as there was some crud hidden away somewhere, and it decided that after the clean it was a good time to come out.
Trying to suggest the cheap, easy basic things first before you get too creative.
Let us know how you go and what you find.
Trains
edited to add.
ive also come across badly pitted water pump impellers that had lost most of their blades, and would just work when normal running, but wouldnt cope with highway running high rpm. as they would cavitate, and not pump the water up at speed. Its a long shot, but not worth looking at just yet.
Save the Whales......Collect the whole set.
My Wife Can't Shear..............But You Should See Her Crutch !
and 4 packs of oil from yota cant make it come good 
) and i also know mine is overfueling cos the pumps worn...