1992 Mazda 626 revs

Engine Performance problem
1992 Mazda 626 4 cyl Front Wheel Drive Automatic

hi my wifes car has just had another motor fitted and we now have a problem with revs, on idle it will run at around 1500rpm then as it heets up it nearly stall at about 250rpm then it will jump to around 2 grand then drop back to 1 g then down to neer stall then up again this happens with a slight hesitation at times 00 it also when neer stall it seems as if it is getting 2 much fuel as it smells rich, it starts no prob and u can excelerate without any problem it has stalled on the road and took a few trys to start seems like it flooded