When the boiler calls for heat, quite often the blower fan will come on for 20 sec. and flame will appear. After the flame goes out, the fan continues to run until I have to shut it off manually allowing it to reset itself to reignite. It may go for days with no issues then not work properly again. Several parts have been replaced in an attempt to correct this problem, but I am feeling that the technicians are just randomly replacing parts without dealing with the issue. HELP!