Hi!
I am sending a short theory behind max. likehood method and its adaptation in comparing curves with given errors. I believe this will be helpfull - it seems to me as the best way that does the trick: - it takes particular errors into account - lets you estimate errors of the fit - as well as contours of confidence when making a grid. This is well tested in my own soft so I hope should work in this case as well. In case of questions - I would be even more than glad to help.
regards Michal