Vauxhall Astra vehicle cuts out when going down to low gears almost full power on accelerator pulls it back

Difficulty starting seems like a lack of fuel. Away we go no problems then I slow down at junctions or lights and I can lose all power but if it is a quick slowdown full throttle on the accelerator will allow me to keep going with kangaroo juice until we get up speed and increased gears.
When I stop the car after seriously revving the accelerator it sounds lumpy as though it is struggling.