It could be a vacuum leak, I think. From what I've seen, a bad BCS wouldn't let you get boost at all, or it would flutter a lot. And the PCM demanding boost dump, you would see the boost peak at normal before it drops off, then flutters just a bit. But I haven't seen everything, I'm sure.
Man... I miss my regal now. i would jump on that for the price. The doors and such can be painted. I would also ask why the Airbag cover is up. I see no heated seats, probably not a monsoon sound system since the deck is removed. Interior looks very clean. 97 was also the first year for the car, so not sure if any issues from that. I know that there were some oddities with the 97'* that the 98+'* didn't have. That one looks like it escaped the exterior mounted antenna. Not sure what the other ones were. Check www.regalgs.org fir more info on it.