Use low pH buffer with poly-ols plus TMB

I would suggest that you try some of stabilizers for antibody conjugates from Surmodics. These products works quite well for Ab-HRP conjugates and shelf life of 12 - 18 month should expected.

Surmodics will be a good Choice.  Mayur has used it, he can give you more info.

My suggestion is Surmodics........

I'm testing the stability of a monoclonal antibody conjugated with HRP produced in my institute and diluted in Stabilzyme (Surmodics); some aliquots are stored at +4°C and some at +37°C. The conjugate is stable and works well even after 5 months of storage at +37°C.

You can find the best which is also in several commercial ELISA here: http://www.seramun.com/products/stabilizers/stabilizers-for-hrp-conjugates.html

Thanks a lot for all your suggestions...