G'day Mate,
If this was a new engine, & now jsut run in, have you checked your tappet clearance, they do bed in, after a while & may need to be adjusted, to get back to consistant running, both full throttle & idle.
When they bed in, the valves don't open as much, ie, the gap gets bigger, & you won't get a good fuel charge, or a good exhaust. This causes the engine to load up at idle.