I think I’ve finally solved the mystery of these short periods of reception on 87.9 Mhz which lack the normal selective fading of some form of skip or the slow fading of ducting.
The clue was that while I was stopped at a light, all of the sudden the Howard Stern show came on. Howard Stern isn’t broadcast terrestrially anymore but does have a satellite radio show on SiriusXM satellite radio.
It occurred to me that what I might be receiving on 87.9 Mhz is peoples SiriusXM radio converters. That explains the usual short duration of the reception (they drive away).
Doing some research, I found that indeed the converters are tunable across the entire FM band but 87.9 Mhz, owing to it being clear across most of the country, is the most widely chosen channel.
I also found out there had been F.C.C. actions with regard to these SiriusXM converters being over-powered and causing interference to legitimate stations. So it all fits, not real DX but bleed from satellite down converters.