Cells frequently lock up in Jupyter Notebook

Hi I am not sure this is the best place, but I have been having a recurring problem in Jupyter Notebooks using Julia (but this seems a generic problem).

I’ll be working (happily?) in an Notebook, and then all of a sudden I can’t do anything with one of the cells (other cells are fine). On such occasions a cell cannot be edited, copied or executed. Also, I notice that the highlighting on a cell when this happens has turned blue (whereas all the ones that still work highlight as green).

Sometimes I can fix by changing it to ‘Markdown’ and then back to ‘Code’ again, but sometimes I have to just create another cell and retype, which is time-consuming and annoying.

Has anyone experienced this? I searched on here and didn’t get any hits.

BTW, I’m on a Mac, JN version 6.0.3

Thanks for any help.