Crash in M4 when making a custom palette

Each time I try to make a custom palette I get about half way through [c. 15 symbols added] and M4 crashes. This happened yesterday and again today. Hope it can be fixed? I saved regulary. After the crash I re-opened and the custom palette had disappeared.The palettes I had 'hide' were again all visable. I am working on an orchestral score. Thank you.