[Histonet] precipitaion of DAB on top of tissue

[Eva C Andersson]

Hello Everyone,
I have encountered a very annoying problem. 
During my DAB step it is precipitating on top of the tissue. At first it seems like black dots floating around on top of the tissue but now it has started attaching itself and it won't go away with washing. There is nothing forming in the tube the DAB is prepared in but only once it has been placed on the tissue. It happens regardless of if I do HRP, LSAB or ABC. I have also tried three different DAB kits and checked the pH of my TBST wash buffer. The only things that are the same for all the times this has happened is the H2O2 from Dako, Protein Block from Biogenex, TBST from DAKO and Antibody diluent with background reducing component from Dako.
Would appreciate any suggestions you may have. 
I have also asked the people who cut my section and they say they have changed nothing in their procedures. And it happens with totally different tissues (kidney, skin).
Thank you all in advance for your help,
Eva Andersson
Research assistant 
Georgetown University