I think Sine often looks nice. For the absolute smoothest, use something like Sine.easeInOut, it has a fairly shallow curve so it looks nice through the most noticeable parts of it.
As far as moving an 1800 height object, you’re just honestly going to have a bit of a tough time. Try caching it as a bitmap if you’re not already, that can help. You’d also get better results from AS3.
Yeah, I figured it wouldn’t be. Sine should help though (that’s in AS2, right?) as well as bitmap caching. Strong is gonna chop a bit - since it’s very fast at first, and very slow later.