Small Engines (Lawn Mowers, etc.): john deere L120, old john deere, loose power

Question
I have a 3 year old John Deere L120.  That has a Briggs 20hp.  Since new it
will
intermittently back fire and loose power.  I have added fuel drier and cleaner.  
It is happening more and more.  Sometimes does not do it at all sometime
does it often.  I have change were I buy fuel and nothing helps.  When
running correctly or low on power I see no fuel in filter.  I have 25 years
experience at a high line auto dealer but no real experience with small
engines.  Any advice would be great.
Thanks Todd

Answer
My first thought is the seat switch doesn't have a good connection.

My second thought is why are you thinking so hard.  This tractor operates just like any other engine. V-8, V-6, inline 4. They are all just engines.  This just a V-2.  It works just like every other engine or car you have ever worked on.