Some clarifications please Paul. When you say you have checked the timing, you checked the cam belt sprocket timing mark against the exact vertical from the cylinder head? And you are sure the cam belt hasn't jumped?
When the engine begins to falter, have you tried restricting the air inlet, or slowly pumping the primer, to see if that improves the situation?
Did this problem arise suddenly, or gradually?
Have you checked with a known good spark plug?
Have you checked with the muffler removed, in case it, or the spark arrester, is blocked/obstructed?
My first guess is that the fuel pump may not be up to scratch, but we can discuss that after you clarify those points.
