how can i fix my renault scenic 1999?

my 1999 renault scenic 1.4 starts and drives but pulling out of junction going into second gear it loses all power but doesnt cut out if i keep applying gas it makes a thud and continues as normal it has just passed emissions for mot and doesnt have any problems when at 50/60/70 mph when the engines under full load i have changed the fuel filter and changed the plugs but this has made no difference, i have noticed however that when the fuel is low it happens alot more it has lost power and completely cut out a couple of times but starts back up straight away

andy7109
March 2012