Have you removed all hair and dust from the filters? If they are clogged, your clippers won't be able to draw in the air to cool them down. And are you keeping them well oiled?

I let them cool/oil every 15/20 mins or so and I'm keeping them as clean as poss.

I had exactly the same thing, with the same clippers, sent them back to manufacturer for service on two occasions and they were always the same.