My AF-S NIKKOR 70-300 1:4.5-5.6 G lens will not focus properly. The focus keeps jumping in and out and the motor has gotten noisy. I checked all function controls to ensure nothing got knocked out of setting. I use a D90 and tried a different lens, which acted normally. I attached the malfunctioning lens to my D80 and it continued it's erratic actions. Therefore I believe the problem is with the lens, not the camera.

Your lens is a cheaply-built kit lens intended to be sold as part of an initial body and lens kit.

It's not built badly or with especially poor quality materials, but it's certainly not to the same standard as their non-kit lens offerings and so is not as long-lasting or robust as regular Nikon lenses.

In your case, the focus drive mechanism has broken, possibly due to one of more motor pinion teeth being stripped. If the lens has not been dropped or otherwise misused and is still under warranty then return it to a Nikon service centre for a free repair. If outside of the warranty, then in most countries Nikon offer fixed price repairs, but you'll find it more economical to simply buy a replacement lens. What's failed is not suited to a DIY repair

You can commonly buy new ex-display examples and definitely fully-functional used examples of your lens for a less than the typical fixed repair price. As you don't state which country/stat you live in, I'm unable to provide example prices.