I first noticed it after I left the tent after finishing that Mother/Child quest, and I'm 75% sure it wasn't a problem before I got the lamp. I'm definitely sure it wasn't one before fighting Kalah. As for older saves, I can check, but I don't think I do. Although I'm not that far in, so I suppose I could restart and play up to there, to see if it happens again.
Edit: OK, the earliest save I had was an auto-save when I was leaving the tent after speaking to the mother, after I had already gotten the lamp. On a lark, I also tried removing Minsc from the party and having him rejoin, but that didn't fix it either. I'll try replaying it up to that point, to see when exactly it started, if it'll recur at all.
Edit2: OK, I've narrowed it down exactly. It happens after the dialogue with the Amnish guard in which you get the Reputation and Virtue awards for finishing the quest. And, after trying that dialogue without the Virtue mod, there is no problem, so it looks like it's something to do with it. I might just leave it uninstalled for now, but it's probably something that needs to be looked at, although I'm sure it's not that common, or it would have shown up before now.