I recently had Adobe CC installed on my work computer, and Illustrator CC behaves very erratically. For example, in order to view a file I have opened, I must demagnify it to 50%. When I select an item on a page, sometimes it shows me the dimensions of the object and sometimes it doesn't. Also, I can rarely deselect all: I must select another item. Lastly, if I change the swatch color of a vector item, the change does not take effect until I select another item. Many other things happen that I won't detail here.
How do I get it to behave more predictably?
Have you both the font (families) Verdana and Tahoma installed and active?
Those should at least take care of the hesitation.
For a wider range of strangenesses, you may consider which parts of the following general list may be relevant (the one already mentioned is included in 5) on the list).
You may try the following (you may have tried/done some of them already) and see whether it helps (the following is a general list of things you may try when the issue is not in a specific file; 3) and 4) are specifically aimed at possibly corrupt preferences):
1) Close down Illy and open again;
2) Restart the computer (you may do that up to 3 times);
3) Close down Illy and press Ctrl+Alt+Shift/Cmd+Option+Shift during startup (easy but irreversible);
4) Move the folder with Illy closed (more tedious but also more thorough and reversible);
5) Look through and try out the relevant among the Other options (Item 7) is a list of usual suspects among other applications that may disturb and confuse Illy);
Even more seriously, you may:
6) Uninstall, run the Cleaner Tool, and reinstall.
You should install both Verdana and Tahoma: their absense confuses Illy and makes her hesitate, or even give up, or faint.
They may also take care of the other strangenesses. If not, it is time to look into the list of other things.
Here is a list of threads about them.