First of all, I would just bend the finger on the sear spring to tighten it up a bit.
If it still occurs after that, then I would go for an Ed Brown hammer and sear and disconnector and replace it as a set. I don't think you will get that for $40.
More like $90, but it will be a long time until you would need a replacemen
http://www.edbrown.com/cgi-bin/htmlo...8509019593060?
IPT