slipping clutch

Ever since i replaced the motor in my 1977 series three 88 about 6 months ago the clutch always starts slipping when I drive it for more than about half an hour or when the motor heats up. I'm sure the clutch is in good condition. I figured that if the clutch was worn out it would be slipping all the time. Could it be a clutch adjustment problem? Any advice?? If i can sort it out without having to pull the gear box I'll be real happy!!!

change oil!