I wonder whether the no fuel in the filter is either "normal", or a herring rouge. When it stops, we need to find out whether there is petrol in the carburettor. If not, then we start to get into the realms of fuel vaporisation perhaps. If there is petrol in there, then I think you need to look at the electrics. Oh no here we go again. Kit car rule number one, if there is a problem with the electrics it will be an earthing fault. Rule number two, if you can't find a fault with the earthing, then it is definately an earth fault. Rule number three, if rule one and two have been ruled out, it is definately an earthing fault. Why 20-25 miles though? My money really was on the vacuum in the petrol tank theory, but if you have ruled that out (which might have explained why the petrol in the filter is being sucked back), are the distributor/distributor cap/points/condenser/rotor arm/plug leads/plugs OK? Change the rotor arm anyway. Condensers are a black art and could easily be the source of your problem, as are plug leads and distributor cap as they can deteriorate electrically and look perfectly good from the outside. I had to change to "cold" plugs as my engine cut out when standing in heavy traffic for more than about 15 minutes, as well as loads of other mods. One thing nobody thinks about is that the petrol we get nowadays is designed (formulated?) for modern engines and our old fashioned engines don't like it much. Just look at some of the classic car websites to see how they get around the various problems, like fuel vapourisation which is caused by the new fuels.
On the earthing front, check your earth strap to the engine, and, thinks, do you have a ballast resistor in the ignition circuit? If so, is it overheating?
I still think it is a vacuum in the fuel tank!!!!!!!!
