I have a 1974 Honda CB360T which for most of my purposes runs well. 
However I expereince a severe power drop at cruising speeds under load. 
That is, on a mild upgrade @ 50 mph in 4th or 5th I'm lucky if I can
hold speed.  If I try to add throttle much past 5000 rpm, power drops
drastically.  Put simply, under load, the engine won't rev past 5000
rpm.  The top third/half of the throttle range is dead.  Standing still
the engine runs fine up to red line (9-10K).

Other phenomenology: at about the point that power loss kicks in, the
engine becomes a little wavery.  That is, at a steady throttle the
engine speed goes up and down slightly.  The bike has about 13000 miles
on it and is in good tune, at least until it starts to balk.

I would appreciate any suggestions as to what's ailing the poor beast. 
My thoughts run toward clogged jets and/or improper spark advance.  I'm
hoping it's not something more drastic since the bike's not really worth
the hassle of any major engine work.

                                        Thanks,