The 18-200 is an AF-S lens and there's no way it should be sluggish. On the other hand, the fact that it doesn't focus properly in 'some lighting conditions' would point to a body problem. The only thing that I can think of in the lens that would cause focus issues in low light would be if it isn't opening up to the maximum aperture.
I've sold and seen literally hundreds of this model lens and it's been pretty trouble free. I'd suggest cleaning the rear contacts on the lens with some denatured alcohol and then doing the same to the body contacts. You're completely AF by wire on this lens so dirty contacts could create an issue. Hope this helps.