RPM dip down when... see the vids
 

The RPM dip down when give it a slight press on gas pedal then back to normal. Also it do the same when turning off the AC. Also I have noted that it Always has a Little vibration during warm up, I'm not sure if this a real issue or related with the first issue.
I don't know what causing that since everything seems fine with the engine, here is the list of repairs done before:
• new spark plugs.
• oil is still good and filter,
• fuel filter is replaced (2months)
• Oil separator with all hoses(6 months)( the hoses is updated to new the new design)
• Oil pan gasket (before 3 weeks)
• 3 differential bushings with new diff oil (before 3 weeks).
• 2 engine mounts (before 3 weeks)

Not e: There is an issue with the compressor (knocking sound) but seems it's not the cause, since the RPM will do that without AC ON, the same will happen with slight press on the gas as I said before.

See vids for more clarification:
Slight press of the gas, the AC is already off.
http://www.youtube.com/watch?v=PBMLIJOotl8

This vid showing how the RPM acting when turning AC off
http://www.youtube.com/watch?v=KM9iwNkOdhw

This vid showing the slight vibration every time warming up. The glass is moving
http://www.youtube.com/watch?v=oSwFm7xrujs


Anybody can determine what causing this, and where to check, I have no codes. Nothing look wrong with hoses or anything.

Vanos seals
 

This sounds like a classic VANOS seals problem. Had this exact issue on my 528iT which has the same engine as your 328 (M52TU). Replaced my old seals which had turned to hard plastic with new Beisan seals and it solved my idle problem, gave me more power thru my normal RPM driving range and improved my fuel economy

read here for more info:

http://www.beisansystems.com/procedu..._procedure.htm