I usually henna once a month at the most. I do it both for grey coverage and to strengthen the hair. After I henna, it usually needs a deep condition to get back to normal and even then can take a few days to start acting right. I've not had any issues with this routine for the 2+ years I've been doing it.
Have you tried going longer than a week between henna treatments? My hair grows really slowly so there wouldn't be much change over the course of a week. But if yours grows that quickly that you need to recover the grey on a weekly basis, I would suggest doing a deep condition after each treatment, and try to lay off the protein in your other products. Alternatively, could you do tea or coffee rinses to get grey coverage as well without the strong effects of henna?
Hope this helps!
3a, medium texture, low porosity(?)
Poo: KC Come Clean
Rinse-out Condish: Jessicurl Too Shea
Styling Products: KCCC, HETT/HESMU, Joiwhip
Weekly Treatment: Jessicurl Weekly Deep Conditioner
Monthly Treatment: Henna
Hair Pics (most I've hidden from public view but if you want to see more, PM me and I'll open up the security on them): http://flickr.com/photos/eponaaran/s...7610671796089/