The burner is clean and has been blown out. I have disconnected the gas line from the valve and get good flow. The thing that gets me is that it burns great and then fluctuates and diminishes down to where the flame goes out. The air control that can be adjusted seems to be correct since when the flame is burning strongly, it looks great. All this is clean and free of obstructions. I have also blown out through the flue to make sure it is free of obstructions and it seems to be venting properly when the burner is burning normally.
Stumped-- Mick