Welcome to CHR, lots of us old farts on here that have ran across almost all the weird little things an internal combustion engine can do! Just wondering what you are using for a breathing system on the engine, are you running a PCV valve or just breathers on the valve covers? Have you tried a leak down check on the cylinders on the left bank? Have you tried setting the intake in position with no gaskets to see what the gap is between it and the heads? Got to be something weird going on there, doesn't really make sense that it would only do it at idle!