error code p0170

hi.... i have a problem with my 328I (pink lady)..... my engine management light keeps switching on and i am constantly getting the code P0170, when this happens more so when sat in traffic or sat with car on tick over anywhere the car starts lumping and shaking and runs really awful etc sluggish acceleration, and this happens whether the car is at temperature or not, the engine light has recently just started switching on randomly while driving, but as soon as i plug in my code reader and erase the code it runs lovely again but never lasts long, it is driving me mental now, just would like some advise as i havnt got a clue about cars........ please help, many thanks