Marcus, You have fuel starvation. I know you have cleaned everything, but the Countax fuel tank was designed by a cretin. It has a very small bore exit which sits at the very bottom of the tank and blocks easily. To test this, turn off all electrics, clamp the fuel line just before the fuel filter. Detach the filter, unclamp and start drainig into a jar. There should be a free flow/constant dribble of fuel. If it drips then stops, there is your problem. You will have to remove (Battery &) tank - not easy- and thoroughly clean that exit pipe and wash and dry the tank eg soap and water. In future use a filtered funnel to refuel. This worked for me- Good Luck!
Roger
July 2009