Your problem may be different, so try the simple things first.
If you think how small the air supply needs to be controlled by the throttle body butterflies and any by-pass air jetting at idle, any small air leaks can make a big difference.
You really need to get air down there like I did and perhaps spray soapy water around the body flange. There's another step that's more complicated but tells you even more. I think you could block off the emission return hole in the throttle body, leave the body in place, remove the airbox, and make up an airtight adaptor to fit a cycle pump/air line and plug it into the top of the body. Then when you spray soapy water around you get to see any air leaks including the throttle butterfly spindles. Since my motor isn't in the frame, I don't know how difficult this is. If you get massive leaks your valves are shot!
BMW haven't allowed a lot of rubber on the manifold O ring seals, given that aluminium expands a lot when hot (and ABS plastic doesn't) - I can see a possibility for intermittent idle problems. But as I said, each issue could be different and checking the obvious like throttle cable and sticking butterfies comes first. My emission return hoses were badly perished and split, but their condition hadn't caused any idle problems (they aren't connected after the butterflies).
I tried a K&N filter for a year. Now I've been inside and seen the mess around the throttle bodies, I shan't use them again. The brown sticky oil they put on the filter had migrated into the throttle bodies, allowing all sorts of muck to stick to them. It took me a couple of hours to remove the whole lot and clean everything up.
I have the large spec'd free play in my throttle cable, checked lock to lock, and I can actually hear a positive click as the body spindles hit their stops - and I've got the springs relaxed. Just by listening you should be able to hear the right sounds with the motor off.
There's some other interesting electronic things on the common butterfly spindle which could also be looked into. There's the TPS we know a bit about (DON'T remove it or mess with it!). It has 2 variable resistance ranges, a fine one for the first range of throttle to about 1/3, and a second resistor more coarse going 0 to 100% throttle. There's also another jobbie, which is just a switch, its one way when the throttle is on the stop, and the other when the throttle is moved off it. Although I've measured a few things on the wires, I haven't fully studied the purpose of the switch yet.
Sorry if this is a bit more complicated than squirting cleaner around your throttle bodies.
Never pay again for live sex! | Hot girls doing naughty stuff for free! | Chat for free!