Check for DTCs, even if your MIL has never come on.
Check the Smooth Idle Compensation in the data blocks, see if you notice a big change with one or more of the cylinders when it happens, if you can catch it in the act.
Check the KW in the data blocks. I like them all set at "0.00" but with the BRMs I have found they run a wee bit better with them towards positive, maybe 0.50 to 1.00 at the very most.
Wiggle the engine harness around, where it goes across near the starter, to the body wire holder. See if that makes any change while the engine is idling.
Check all the temp sensor values in the data blocks (fuel, coolant, intake air), to see if they all remain the same once warm and don't vary and move around with quick, rapid changes.