Does this happen when the View > Display Performance is set to High Quality Display only? or also in Typical Display?
A colleague described a similar problem a while back to me, and her document used Anchored Objects, and had Optical Margins enabled for the text frames, and from memory the issue was less severe when for Typical Display performance, and it might have even disappeared completely when the Optical Margins were disabled.
I have no idea if your document meets the above formatting, but if it does it might be worth a try.
Thanks for reply Cari, I've tried everything including your suggestions, e.g. reset prefs, reinstalled fonts, different Keyboard, different user login etc etc . I've even reinstalled Adobe suite on a new PC. Still happening. After much discussion with colleagues who are all having the same problem we now suspect its more an operating thing, as the combination of ctrl, shift, alt triggers the disappearing text. I am gonna use the menu instead of shortcuts for a few days to see how I go.