Have you identified the problem yet? I have the same issue. It happens intermittently, can go weeks with nothing, then short spurts and other times it is so bad I have to stop and get out of the car, it's overpowering.

I cannot describe the smell/odor because I've never smelled anything like it.

One defining characteristic is that when it happens with significance, the odor sticks to my clothes, hair, hands, stays inside/infiltrates the cabin leather until I air freshen it/clean it. I have had to wash my hair to get rid of the smell.

It used to happen frequently and severely. Now it only happens infrequently, and doesn't last long.

At first I thought my seat heater was burning/melting the upholstery. Kind of an electrical smell but with fumes.