I'm running MS Office 2011 14.7.7 on High Sierra (MacOS 10.13.6) on an SSD formatted as HFS+ and don't have more than the ordinary occasional crash with it. I mainly use Excel, occasionally Word, and once in a blue moon PowerPoint. Excel seems to become unstable once I have something like over 50 spreadsheets open so I try to close as many as I can when I don't need them any more. Other than that I like it better than the "upgraded" versions.
Are you getting any error messages when they crash? Might help point us in the right direction. Rebooting the Mac in Safe Mode will clear out some caches which may cause some problems.
You should make a backup before you do anything. I found the following instructions on the web.
I’ve had occasion to move MS Office 2011 to new setups a number of times. I simply keep copies of all the files it places in /Library and ~/Library (some of them during installation, some on first run when serial number is entered to register it) and it has worked fine. (Or nearly; in 30 years on the Mac I’ve never used Office until last summer when I needed it to work on a complex document I was helping a friend with. I found it an unpleasant experience – though that may have been partly because I was using it in macOS 10.12 Sierra. But it did work.)
Below is a list of these files. I expect the crucial ones are probably the “licensing” ones, also maybe the “Office Registration Cache”; but I’ve never experimented, I just copy the whole bunch, along with the Microsoft Office 2011 folder from /Applications, which makes it unnecessary to run the Installer at all.
Code:
/Library/…
Application Support/
Microsoft [folder]
Automator/
[98 items with extension .action]
Fonts/
Microsoft [folder]
LaunchDaemons/
com.microsoft.office.licensing.helper.plist
Preferences/
com.microsoft.office.licensing.plist
PrivilegedHelperTools/
com.microsoft.office.licensing.helper
Spotlight/
Microsoft Office.mdimporter
~/Library/…
Application Support/
Microsoft [folder]
Preferences/
com.microsoft.autoupdate2.plist
com.microsoft.DocumentConnection.plist
com.microsoft.error_reporting.plist
com.microsoft.office.plist
com.microsoft.office.setupassistant.plist
com.microsoft.outlook.database_daemon.plist
com.microsoft.outlook.office_reminders.plist
com.microsoft.Powerpoint.plist
com.microsoft.rdc.plist
com.microsoft.Word.plist
Microsoft/
Office 2011/
Custom Dictionary
Microsoft Office ACL [English]
Office Registration Cache
OfficeSync Prefs
OLE Registration Database
Word Font Substitutes