Jewellery Pix - blurry results: change lens (get a macro?), camera (get a D7000?), strobe or else?

Hi,

So following this thread - http://forums.dpreview.com/forums/post/50059230 - I took everyone's advice...but am still getting unsatisfactory results.

As a reminder and with an update, my equipment is the following: body Nikon D90; AF-S Nikkor 70-300mm F:4.5-5.6; AF-S Nikkor 18-105mm; Nikon 50mm f/1.8D AF Nikkor; tripod Manfrotto 055xprob with 808RC4 head; sb-600 flash; remote ML-L3; lightbox 24" square & twi side strobed (light 15W, 1500K each).

I have tried varied combinations but still end up with pictures that are ok but not razor sharp. I now use a lightbox with my 18-105mm lens, at 105mm, shoot all manual with manual focus, strobes on the sides of the lightent (but don't use my SB-600).

I suspect my lack of excellent results to be caused by failing skills, but still think my equipment might be ill-adapted (e.g. no macro lens; D90 simply not have "what it takes") as well for tha quality I am aiming at.

Again, I would welcome advices, thanks!

Below are pictures I just took ---

D90 / tripod / lightbox 2 side strobes / 105mm / 1/60s / F;5.6 / ISO 560

(1) On this one, not razor sharp, the "white" background is still visible - I thought the purpose of the white box was to somehow "eliminate" these? Should I use white sheets of papers instead? Or eliminate it in PP?

D90 / tripod / lightbox 2 side strobes / 105mm / 1.60s / F:5.6 / ISO 560

(2) Here, we can see that the DOF is super shallow. Maybe too much.

Same as above, except 1/80s

(3) Still the background problem, but more importantly, the sharpness that is really not great.

Same as above, except 1/80s and ISO 640

(4) The problem of sharpness here is visible too, though less.

Same settings.