Are you monitoring the change in absorbance at 340 nm unpon oxidation/reduction of NADH/NAD? It's possible that at high imidizole concentrations you are seeing absorbance in this wavelength region. It's best to use freshly prepared imidizole since imidizole can be oxidized and show more UV wavelength absorbance. (That's why it turns brown). If you are not saturating your dectector (i.e. your OD