A few items can cause this, most common one is as BBrown said something blocking the air intake or being sucked into the air intake.
List is endless,
fake plugs,
internal corrosion in the plug caps,
plug caps loose on the leads,
dirt in the carbs being sucked into the main jet,
very worn Throttle Positioner.
There are a few rare cases of a faulty faulty rectifier/regulator and rarer again a faulty Ignitor.
A few cases of overly restrictive air filters, HI Flo in particular had this issue.
Clogged Air filter
over oiled K&N Filter,
Dirty fuel filter,
blocked carburettor vents
Unfortunately no quick and easy solution
Take your pick, it is a matter of ruling out each item, no easy way
List is endless,
fake plugs,
internal corrosion in the plug caps,
plug caps loose on the leads,
dirt in the carbs being sucked into the main jet,
very worn Throttle Positioner.
There are a few rare cases of a faulty faulty rectifier/regulator and rarer again a faulty Ignitor.
A few cases of overly restrictive air filters, HI Flo in particular had this issue.
Clogged Air filter
over oiled K&N Filter,
Dirty fuel filter,
blocked carburettor vents
Unfortunately no quick and easy solution
Take your pick, it is a matter of ruling out each item, no easy way