Considering it is now 2009, This is a bit of a necropost....and I apologize for that.
But due to my frustrations and the fact that I found this thread which seems to describe the exact problem that I am experiencing, I thought I would bring this up.
We ordered a (boom?) converter for my brother for christmas from amazon, which worked beautifully. The joypad is steady as a rock.
We quickly ordered two more of them - one for me and one for my sister (this time from an EBAY supplier, but I'm pretty sure it is the same model as the markings on the controller are the same). Both of these suffer the exact problem described in this thread - When the joystick is moved along the y axis it oscillates irradically, making it impossible to perform certain functions, like aiming in goldeneye. We've tried it with 3 original N64 controllers, two of which I would say are in great condition. We also tried it with a brand new knockoff controller. We even tried it on three separate computers, and always came back with the same result - the first one we bought works great with all computers and all controllers, and the second two exhibit the exact same, serious problem on all computers and all controllers.
Are these just quality control issues, or is there some other difference between the adapters? The new ones look exactly the same in every respect except for the fact that they come in a translucent casing rather than an opaque black casing that the first one we ordered did. It just seems strange that we would get two lemons that experience the exact same problem.
/rant