what's wrong whith my micra?

I have a 95 micra which runs perfectly when cold but after about 15 miles starts to judder as if running out of fuel.I have checked the fuel supply(taken off the fuel filter hose on the delivery side of the filter )and the petrol flow is strong and constant,cleaned all the engine mltiplugs and put new plug leads on,cleaned dist. cap and rotor.Could the coil be breaking down?