Try a bit of aerostart spray when it starts to die and if it keeps running, it's a fuel problem, if it still dies it's a spark problem. Also check your oil level and make sure it is full since the low oil cut out may be kicking in. If it is full, try disconnecting the cut out wire, the float may be stuck in the sump. If it keeps running on the aerostart, check the needle and seat for a possible bit of crud stuck in it. If all else fails check the valve clearances, they should be 2 and 3 thou I think from memory for the Chondas.
