The DISA system is the "3 stage manifold" that people refer to it as around these parts.
The 1st adjuster unit is the one closest to the throttle body. The concept behind them is to stabilse the air pressure behind each valve and allow it to make more power in the higher rpm range.I had a good illustration that explained the system but cant find it atm.
It shouldnt be dirty from carbon build up, unless of course you dont have the air filter on properly or havent changed it for a long time.
Its has most likely just failed and needs replacement. Should cost about $200-300 for the part. Talk to Stu at bmrautowerkes to see if he can get the part cheaper for you than BMW.