Show us your mandrel mate, everything wrong here IMHO. There are too many unknowns, eg how the mandrel is held either end, if you use a point contact to live centre is the point 60 degree or not fitting correctly. Sounds like time to go between centres, eliminate the use of a mandrel, see how you go then.
Is the alignment between the headstock and tail stock accurate (tested with a dead centre in the headstock and tailstock meeting spot on, this tested by lightly interfacing a thinnish feeler guage between the meeting of the centres. Out of alignment can be in any direction 360 degrees.
Just a few things for starters. Show us pics of your lashe etc, think about a mandrel saver or making one.
Peter.
