It could be a plethora of things, I'd start by dumping the transmission fluid and inspecting it. It could also be the clutch material breaking down once it gets hot. This can also happen to brake pads if left unused for many years. I'd start to snoop around the clutch master/slave and do a quick visual on those and the hoses as well.

Be sure to replace the spark plugs, cap and rotor. A PCV valve is also a good idea. While you're at it, grab some vacuum lines and replace whatever you find that's cracked/broken/missing.

Did you already change the belts and hoses? Now would be a good time if you didn't.