So, I've had this issue for some time, and several Google searches over the last several months have shown that others either are having or have had the same issue. (Though to date I've not found a solution.)
My issue is that in Safari, (Version 11.1.2 (12605.3.8.1)), on a 2013 MacBook Pro, I often have several tabs open, sometime 10, maybe 15, or more. Of whatever number of tabs are open, 2 or 3 of them will be Google tabs, usually gMail, Google Calendar, and often a 3rd tab for Google Drive or possibly Google Sheets.
Of all the open tabs, ONLY the Google tabs seem to have problems.
At some point, most or all of the Google tabs will stop responding, or crash, or whatever the appropriate terminology would be. (I think that the messages that pop up may say the the page has 'timed-out', but I'm so accustomed to this that I have not paid attention to the message for months now). I usually just close the tabs and open new ones, as that seems to be my only option.
May be worth mentioning that I usually have both Safari and Chrome open at the same time, in different 'desktop' windows on the MacBook Pro. Personal stuff in Safari, and work stuff in Chrome, for the most part.
If anyone has any insight into how to resolve this, it would be most appreciated.