I noticed this happens if you try and select a value that is already selected.
You can work around this with a pre-check:
if(_sel_ddlPeriodFromDay.GetValue("value")!="1")
_sel_ddlPeriodFromDay.SelectByValue("1");
or maybe use a try catch?
try{_sel_ddlPeriodFromDay.SelectByValue("1");}
catch{}