How many fonts do you have installed? Try to work with lesser fonts.
Can you please try the following workaround and see if it resolves the issue?
Navigate to Preference(from edit menu) ->Advance Type -> Type Contextual Controls and disable the first preference “Show for Character alternates, Fractions Ordinals and Ligatures" or turn off both the preferences.
Also if this solves your problem, can you please mail me your document at firstname.lastname@example.org
Thanks Aman, that has solved the issue.
I also checked it on another document that has the same issue and it's worked there too.
I unchecked the first option only by the way - the 'Show for Character Alternates...'.
Thanks again, you have eliminated what was becoming a very frustrating problem.