Mazda Repair: engine cutting out, o2 sensors, mazda 626

Question
i just replaced the engine in my 95 mazda 626 car idles fine but is really sluggish in low rpm and cuts out around 2000 rpm if and when you get it past that point it runs perfect and strong. the car appears to be running really rich replaced all vaccum hoses and followed diagrams to the T. i'm kind of leaning toward the o2 sensors but not sure (it's the only thing i didn't replace) timming is set correctly new belt installed. please help

Answer
it's not the 02 sensors, said you replaced the engine? was it a junkyard complete engine? if it was, seen injectors gum up from sitting around, also pull the vacume line off the fuel regulator and see if any fuel is leaking into the line, also remove the maf sensor and look closly at the fine wires to see if any crap is on them. i think it's one of those 3 things