Tony, there are some common problems and solutions, don't know if any of these are the cause of your problem.
Carbon buildup in the combustion chambers.
Sludge buildup in the crankcase due to infrequent oil changes.
A poor state of tune (old plugs, weak plug wires).
Do a google search and you will find several suggestions for causes and corrections, there are even special "snake oils" that you can use that are supposed to correct the problem.