Roto Brush causing crashes

I'm having multiple crashes whilst using the Roto brush at different stages, Painting, Freezing, Rendering sometimes even as the project opens difficult to predict

no two times the same.

system is an iMac 3.5ghz i7

32gb of ram

NVIDIA GeForce GTX 775M 2048 MB

I'm working with prores 422 footage on clips no longer than 20 seconds.

and 100gb cache  on an external hd

any help would be greatly appreciated

E