can't answer RBA's question, i've not seen that error and i use it all the time. however, David's issue is very likely that the swap files that PI creates are be filling up your "temp" directory. one is created at every step of the masked stretch. so, after doing a masked stretch i usually clone the view and close the original - this will delete all the swap files created during the masked stretch. i don't know if these files are just sitting on the disk, or if because of the nature of how PI uses them, they are mapped into PI's address space. that might cause your VM system to slow down. so you might try this instead of quitting PI and see what happens.
RBA, is it possible your disk filled up while running masked stretch?
