The first question is what clutch do you have fitted? Second question is how old/worn is it? Has it been OK and for how long? When does it slip? - full power in high gears is the usual. If it is slipping when hot it is probably borderline anyway and the heat either reduces the friction of the lining slightly or it causes expansion in the clutch cover which eases the clamping pressure on the driven plate. The clutch max torque rating may be very near the engine torque output.
Check also that the clutch arm when released is not hitting a stop of some kind. As the driven plate wears the clutch fingers move outwards. This will push the clutch arm away also. If the arm meets a stop point such as the edge of the bellhousing aperture this will cause a pressure on the clutch release mechanism which will cause partial release and hence clutch slip. The slave cylinder can bottom out causing a similar effect. Check for free play in the release mechanism - there must be some - even a few thou.
That's probably the culprit--a misadjusted clutch. You should have noticeable free play at the pedal (perhaps a half-inch of pedal travel) before the clutch pedal becomes 'hard' and you start to manipulate the clutch. If your clutch starts to disengage as soon as you depress the pedal, then it's being pressed all the time. That will wear out the throwout bearing and will also sizzle the clutch, as it will be slipping under heavy load.
If you've been driving the car for any length of time like that, the damage might be done, but it's worth having a look and backing off on the adjustment if necessary. FWIW I just had this same issue with a VW Scirocco; just a few turns on the clutch cable adjuster returned the necessary free play and the clutch slip disappeared.