loss of power

hi guys, ive recently bought a 2001 e46 320i 2.2lt... now and then i lose power.



if the fuel gets low the power drops. if the car stalls out.. the power drops and sometimes if im parked up for a period of time and try to pull away.. the power drops.



this isnt a constant issue and if i switch off the engine for a min or two, everything works fime again and i have full power. its pretty annoying.



any ideas?