Well it really does sound like insufficient output from the fuel pump/filter, doesn't it? Is it a mechanical pump/carburettor?
I think you're going to have to run it till it stops, then take the top off the carburettor to see if there's petrol inside. If it seems low, then continue along your previous course of filters and pumps, checking and swapping - try leaving the petrol cap off as an experiment to see whether the fuel tank is perhaps becoming 'vacuumed' (sorry, I mean the opposite of 'pressurised').
If there is petrol in there, then it must be some strange ignition fault - perhaps an early warning sign of the finned electronic module dying, though as I recall you have replaced this?
I have another idea, connected with the low air pressure of Johannesburg (OK so you're in Gauteng ("how-teng...?") which is still quite a long way above sea level if I remember correctly...) The decreased air pressure lowers the fuel vapour point (as does cheap petrol - now there's an oxymoron). So maybe you are getting a fuel vapour lock scenario? (used to be common with older FIATs and give difficult hot starting etc.) Does the carburettor have a return line to the tank - this is supposed to provide a continuous flow of fuel and prevent overheating of the fuel in the lines near the carburettor. Check that the heat shield is in place under the carburettor. Check the fuel pipes are as far from hot engine parts as possible. Try a different brand of petrol.
-Alex
