Yeah, that's what I thought. I'm sure I could shoot in the same conditions at ISO 100 on my old camera, but I may be wrong. I'm talking bright sunlight coming through a window here, not just a bulb. It's perfectly fine outdoors though.
The lens has a built in focusing motor, but it's relatively new so I doubt it's that. I had this problem with my lens with the D60 too. I don't think this lens is compatible with a motor in the body. I'll give cleaning the contacts a go - didn't think of that.
Light coming from outside is still not as bright as being outside though.
I've used the video mode in very windy conditions and it's fine - the AF motor is really loud though yeah, I use manual focus to eliminate that problem. The continuous AF is terrible anyway. I notice quite a bit of noise in the videos I take - is there some setting somewhere I'm not aware of?
Never use AF in video, DSLRs are not camcorders you use them to get nice shots more than just recording moments. 
You can use a silicon jar opener as a cheap follow focus. Like this.
Also use all manual and set your shutter speed as closely as double your FPS. (if you shoot at 24fps use 1/50, for 30fps 1/60 etc.)
The D3100 is overall not for videography in my opinion, with the video being skippy instead of blurry because it reaches 30fps at most, and the integrated mic with no mic jack. It's just marketed as that because it's a d3000 with video recording and a couple other functions and improvements.
It should at least have a jack for an external mic. Seriously, how expensive is that to implement? (also, rant about it having a proprietary shutter cable connector and no infrared shutter release number one million.)
The video being too skippy could be because you have a shutter speed too fast for the FPS (you get choppy motion like Saving Private Ryan) or simply you are trying to shoot action too fast for the FPS you have set.
Remember that pretty much all movies (a part from slowmo scenes) have been usually shot at 24FPS. (because it was the slowest they could go to keep the costs of film down)
Also for audio you can just use an external recorder and sync in post with the on board audio. (that's a standard for cinema too)
As I said before the problem is expecting a DSLR to be fast an easy like a camcorder, you can get great result if you know what you are dealing with and how to work with it. (which also means knowing it's shortcomings like moire, rolling shutter etc.)