You may well have Rust inside the Tank....many owners hear have had this problem as the rust jams up the Fuel pump, The tank needs replacing as a complete cure, if you do this remember to put 10 lts of petrol in after ....not just 5lts....as the fuel pick up in tank is 10mm above the bottom of tank...have come across this Pump problems hear over the years...the tanks rust on the inside over the long years now...look good on outside. best i can help. Peter