very intermittent, very bad shimmy

I'm done many searches, but cannot seem to find an existing thread regarding this -

Twice in the past 6 weeks I've been driving at about 60 mph when suddenly a violent shimmy starts, like if a wheel weight was suddenly lost, seemingly from the front right tire. I stop, look at the tires to see that there is nothing obviously wrong, then start up again and drive 30 more miles at speed with absolutely no problem. Between these two events, I have driven the freeway with no problem.

It seems with most front end issues the shimmy is consistently present at certain speeds, but that is not the case with this. It is strange how bad the shimmy is, to only go away without anything mechanically changing. Otherwise the car drives fine.

The car was in for an alignment about 2 months ago at the BMW dealer (due to a tire wear issue), and they indicated they did their 'multipoint' inspection, so I would guess if there was any obvious front end issues they would have flagged it and offered to fix it for me (kind of the purpose of the free inspection, I suppose).

I am running snow tires with aftermarket alloy rims. In both cases there was not snow lodged in the rims.

2002 325i with sport package

Any ideas would be appreciated.