We had the same problem (in New Zealand) with our 2003 318i (4 cyl). Car started fine in the mornings but once warmed up would idle erratically at every stop sign or traffic light. We checked the vacuum lines and all electrics but to no avail. The problem went away when we replaced the fuel filter. Shouldn't cost more than about R150 - R200. I think what happens is that the electronics automatically feed the engine with more fuel at startup but when the engine has reached operating temperature the fuel pump struggles at low revs to get the fuel through the clogged up fuel filter. Hence poor idling (hunting up and down) and poor acceleration from standstill and poor fuel economy. Good luck.