lol well I'm sorry to hear that, I get like that every once and a while, especially when last summer my exhaust mani was leaking like crazy and throwing everything off all the time, I did notice that with different temps it will throw the afrs off, but I think I remember it lagging like what you are talking about a while back, I want to say it was before .. oh actually do you happen to have an EBC? if so I remember that the way I had it set up, it was sometimes too sensitive and really lagged like s*** and was just annoying as all hell, I just gave up on that, at least for a while, right now I have the 8psi tuned well.. for the most part, and this summer ill work on the 10psi, I just remember if I didn't have the settings correct it would lag like a b**** and then just spaz out, especially if it spiked and reduced it b/c of the limited I had on it.
if you don't have an EBC than maybe if you have a MBC, maybe the spring is weird, or I remember when my WGA blew on my old msp it was bad like that too, (but I think I remember you saying that you couldn't blow into it when you tested it right?), I couldn't tell it was blown until I manually tested it by trying to blow into the vac line, because it wasn't bad enough where I would notice it on the boost gauge if you could do a datalogg and upload the map with the log we could take a look at it, could help, never know, more info always helps
one more thing, I did notice that you said you have a fully stock msp kit but with FMIC and ssafc, a few things on the airflow could be screwing with your turbo and creating not really lag but more backup, I would look into getting an exhaust on there, it really really opens the car up, it just feels soo much nicer, or if anything just a midpipe is good too, that stock double CAT really restricts flow, and I guess that also means it's on stock boost, so nvm about the EBC or MBC lol, but check the wga again by trying to blow into it, should be rock solid. and also like I said before, most the "over pressure" to something like .5 or -1, try both and play around with that, I noticed that when I first tried to make the begining of the fuel map at like 1-2psi boost a little richer (because for me, the afrs were kinda lean at 1-2psi) it actually made it really rich, then really lean, and when I just leaned it out around 1.5-2psi to like 9.9 on the fuel map at low rpms, it actually brought the afrs down quicker, so I would just play around with the fine tuning in the beginning on the psi range, and watch carefully for that really super rich drop, and a quick lean spike right after that, and move the "over pressure" around until the lag between switching from closed loop to open loop becomes less noticeable, and go from there.
the car takes a tonn of patience, but after driving my Dodge stratus for the winter, I can't wait till i get back into my msp next weekend! lol just take small steps and don't over work it or it will just get worse usually