Erratic idle and neutralizing transmission?

Hello,

Sometimes when at a stoplight or coasting to one, the idle becomes slightly erratic, and the transmission neutralizes. If I wait a few seconds, it goes away and back to normal. Only just started doing this. Fluid is at normal level when hot. It doesn’t smell burnt, isn’t cherry red, but more of a lighter maroon color. Fluid and filter done approximately 2014 at approximately 50,000-mile mark.
Never a taxi, private owned vehicle.

Can this be something other than transmission related (I hope)?

Will adding an additive help or make it worse?

Thanks In advance.