I'm on Adderall. My experience with this medication is that it does not seem to totally prevent me from jumping from task to task. For me it seem to help me set settled and into my productivity zone and stay there. At the end of the day I may not have finished one thing, but I've gotten a lot of chunk of many projects done.

I don't know anything about the Namenda medication. Could you be experiencing an interaction between the two. Like I've started taking a betablocker for my BP and I notice that I feel significantly different when I take both medications than when I just take the one.