Do you have a ground wire kit if not get one.. I have a manual and it normally idles around 700 to 900 and when first got it it would do same thing at like would drop rpms real quick then back up then did ground wire kit and it stopped doing it.
The engine shudder happens to me also. I've talked to other local xb owners and they are having it to. We didn't think it was anything serious, probably some electrical load demand or something and the engine slows or idles back up on its own. At least thats what we hoped it is. Kind of has a dry bearing type sound. Be good to know for sure what it is.