Dell PC 7070 Optiplex Windows 10 Office 365 - DropDown Menu Problem

Good day.
Problem occurs on a Dell PC - OptiPlex 7070 SFF 2020 equipment. Operating system, drivers and productivity programs are up to date and maintained regularly.

When working in Windows 10 Professional and Office 365 Professional, the drop down menus are very slow to open and then when they do, they close immediately not allowing me time to select the desired activity like cut, copy, paste etc. This is a serious impediment to my work.Predominantly in Word and Excel. Less so in Outlook

Regularly use Dell Support Assist and Dell Command Tools to check the system. No issues detected.

Have worked with Dell ProSupport.  Bios updated.  Drivers confirmed up to date.  Troubleshooting does not reveal any hardware issues.

Have run SFC scan now and used DISM restore health repair tools. No problems found.

Task Manager shows only 50% of Memory in use. Disk usage low. No visible problems there.

Have uninstalled Office 365 and reinstalled - several times - does not fix the problem permanently. Might work intermittently for a few hours then back to the same problem.

Have uninstalled Microsoft Sculpt Ergonomic Mouse and Keyboard software and reinstalled from Microsoft download. Problem continues.

Protection Software is Norton 360. Uninstalled and reinstalled it. Same problem.

Have run other antivirus programs - Malware Bytes, Trend Micro House Call. No issues detected.

At chat rooms I see some posts of others suffering with the same problem but no solution to fix the problem.

Have
I would appreciate any suggestions from the group.
Thank you.
Pauline