Motorcycle Repair: Clutch problem on 81 CB400N, cb 400n, clutch lever

Question
I have an 81 CB 400N and have a problem with the clutch freezing. I can be riding along with the revs sitting in the low 5's ok but if I pickup the speed and the revs sit constantly in the high 5's to 6's, generally in top gear, when I come to slow down I find I am unable to pull in the clutch to change down gear, although it will come good after a while.
I originally thought it may have been due to a need for an oil change, but that has failed to fix the problem.
It appears to be happening mainly when the bike is hot.

Answer
Geoff, I've not see anything like this before but there's only three parts to that clutch lifter mechanizm...the clutch lever, the cable and the bellcrank on the right hand side cover.  If the clutch works most of the time, I'd guess the problem is probably down in the right had side cover.  The shaft could be binding up when it get hot, or the clutch may need adjustment and it's not engaging the lifter parts in side the right side cover.  

Regards
Rich