Fuel and power to engine

I have a 2003 model hyundia terracan automatic v6 petrol 4 WD. And I am having a few problems with it. Firstly it can be hard to start. Then when at a stop and put foot down on pedal to go it loses power drops gears and revs then once putting foot right down it will go. On a highway when doing 100 kms an hour it will drops back in gears like it loses power then I have to put foot down to make it get up and go and when this happens it backfires quite loudly. There is a very rich fuel smell at the back of car. I have replaced injectors, cleaned fuel tank and changed fuel filter. I also use half a tank of fuel to 140 kms. I just don't know the cause of the car dropping back in gear when on highway it Wont keep to the one speed. Any help will be muchly appreciated. Thank you Jessica Skinner