If I were you I would try swapping and cleaning both intake and exhaust vanos solenoids. It is such an easy DIY that it is worth trying out. I had a sticky intake solenoid and was having reduced power over most the RPM range and a big flat spot in power at 3000 rpm (when the vanos system kicks in). Both solenoids are easily accessible from the front of the car and all you need to do is get some MAF cleaner to spray them down and compressed air to blow them clean. Once cleaned swap them (both parts are the same, so make sure to mark them) and reinstall. Check out this DIY to get an idea of what is involved:
Honestly shouldn't take you more than an hour tops, the cleaning part is the longest part. When I had my problem it wouldn't always throw an error code so you might be having the same problem. Also, just because your car doesn't have a check engine light doesn't mean your car hasn't stored an error code. If you know anyone who can code ask them to pull your stored error codes using IPNALOAD. It may give you an idea of what is happening.