Gain-on-sale margins – a key profitability for many mortgage lenders – are expected to decline in coming quarters, although well-positioned lenders may be able to outperform the broader market, according to Lehman Brothers analysts. Gain-on-sale margins is the second-largest determinant of profitability – after sheer volume – for lenders that sell production into the secondary market, says a recent Lehman market commentary. Yet predicting margins “can be as difficult as predicting movements in interest rates…