I have a 2 year old laptop (Dell Inspiron 13 5378) that worked fine for its first 18 months then the shift key became unreliable and now stops working. Both shift keys and both alt keys become non-functional. An external keyboard works fine.
Running Dell Diagnostics, I can see the keys are detected at a low level, just not where it counts. I took it to a PC shop and they confirmed the keys work fine under Ubuntu. This is not a hardware issue.
Next, I totally flattened the machine. The shift key works again. Install Office 360 and it stops again.
Next I reflatten the machine, all OK, and I install Office 97. The shift key stops working immediately.
I feels like a Windows driver got updated a while ago and it clashes with Dell and Office combination, but Office 97 is 20yo!