276000 is stellar mileage. amazing!
I was thinking about this problem overnight and saying to my self that it would be good to check the positioning of all the clutch release components, i.e. make sure the release bearing is sitting exactly where it should on the arm, the arm itself, the fulcrum point, where it contacts the rod from the slave as well as all the other stuff we have mentioned.
But at the end of the day, if you have a chipped engine, a heavy truck running on bigger tyres, the point at which all that power has to move all that weight is in fact the clutch. And you may find that is why it is slipping. I personally would have fitted a heavy duty clutch. Not being clever after the fact, just too much history with tuning engines.
Anyway, here's hoping you find it is something simple like properly bleeding the clutch slave.