Afr issue

So it’s been a couple weeks Ihave been having this issue when I cold start car, afrs are perfect once warmed up drives good for a bit but then every time I come to a stop the afrs go lean or rich for couple seconds then back to normal if I go wot afrs are perfect, after pull and coming to stop car goes lean for couple seconds then slowly works it’s way down to normal. I have fbo id1000 injectors walbro 255 had boost leak done changed fp relays, spark plugs all parts have been on car for about 500miles really having a hard time figuring this out. My tuner said that it was normal for car to go lean or rich for a couple seconds as long as it does not stay there. Really just need some help on figuring this out if it’s normal.