My lathe has bronze bearings and as time goes on they do wear slightly, luckily they are adjustable. I can tell when they need adjusting as there start to be problems of slight chatter and definitely problems with grooving and parting. Adjust the tapered bearing further into the taper and the problems go away. My point in telling you this is that if you have poor bearings or poorly adjusted bearings then they can be the source of parting problems.
Martin C