Please help not ready for the scrap heap!!

I am experiencing idle issues with the car upon start up although its not everyday and usually worse when damp outside (typical British weather is always damp)...
It will start and tick over fine no problems as soon as you put it into drive or reverse (automatic) it will start jerking backwards and forwards or rocking for a better word, and will start revving erratically and also will start to feel like its going to stall, feels like its petrol starved if a better explanation. Ive changed the stepper motor and the power steering sensor to no avail, does anyone have any other suggestions or diagnostics as I really love my car but my patience are wearing thin now