I my experience a short stem is twitchier swapped a 100mm out for a 120mm after I got more flexibility and what had felt "unstable" felt much smoother I almost instantly pick up 5-10mph on curvy descents.

IMO if you have to go to a 90mm stem unless you are short on top you are on to large a frame or need to improve flexibility to get the longer stem back on. also IMHO 100-110mm is as short as you should go for a stem and 125-130 is the longest, with 120 being the sweat spot for stem length.