Take a good hard look at the clutch hydraulics - when normal fluids get warm,
the viscosity drops (gets 'thinner') and if you have worn seals in either cylinder
it can start slipping by them. If you have normal miles for the age of the car and
they're still original, there's a good chance that one of them is ready for rebuild
or replacement. Check the flex hose carefully for cracking, too. Mine failed once
70 miles from home, luckily it was mostly interstate driving to get back. Had a
flashback to my Kawasaki riding days for clutchless shifting...