1997 Chevy Lumina Lumina slips into neutral at stop signs/l

This happened last winter, went away for the spring, summer and fall. Then returned during the cold weather.

When I come to a stop, the car is slipped into neutral. I shift it down to L1. Wait to feel it engage, then it's fine. The car shifts fine otherwise. This also is the case when I start it in the morning. It won't engage reverse, until I shift into L1. Feel it engage, then it will engage reverse. All is well until I hit a stop sign/stop light. Fluid level is good. Via a recommendation, I tried an additive. No go.

Thank you for taking the time to respond. God bless,

-jz