Hi,
The originator has only supplied test figures for under 300mA output current.
I doubt if you will get 6A output, especially in protoboard setup.
No performance figures for higher loads.
If you read the article, there is always quotes of output voltages, but no output current of any serious value.
When your output drops, what is the actual output current and voltage and load resistance, as well as the input voltage and current?
When you say "smaller load" you mean a smaller load resistor?
Which means a higher load (current) on the project.
Don't forget POWER OUT < POWER IN.
I suspect this is just a test/experiment circuit and not intended as a viable device.
Tom... 