I had a slight problem years ago when I had the RV8/R154

Car drove fine, clutch always gripped, but over time the pedal feel would just change.

Releasing the bleed nipple and out squirted fluid because the system was pressurised.

Whilst fluid was able to return, clearly the pedal/master was adjusted slightly wrong and it was able to retain some pressure in the line.

So a quick test for you...when the hot slippage occurs, open the bleed nipple to see if there actually is any pressure in there, if so, look to adjusting the master rod.

it's always tempting to adjust this so there is no play...sometimes a little free play isnt a bad thing.