Q: I have a colleague who is moody and unpredictable. Sometimes she'll take days to respond to a time-sensitive request, and other times she's right on it. Sometimes her replies will be snarky, and other times quite solicitous. This does not seem to vary with her workload, and I suspect personal issues are involved. I'm ready to go to her supervisor, but feel I should talk with her first. Any suggestions?
A: Going to her first is a respectful step; timing will be the key.
The inner game
So, how do you feel about it all? Your question is very much about the other person, and not so much about you. Are you angry? Confused? Hurt? It'll be important to identify your reactions so that you can have a successful conversation with her. Take some breaths, and take time to understand your feelings.
It's also interesting that you've drawn a conclusion that personal issues are involved. You may be correct; however, take some time to think about other explanations, too, so that you don't become invested in your own interpretation.
Now, look at the situation from her perspective. If she seems to be aware of her behavior, how does she explain it? Apart from response to requests, are other aspects of her work relationships volatile?
Finally, consider whether it's just you or if other people are experiencing the same pattern. Even if it's limited to you, it doesn't absolve it, but it does help clarify whether the actions may be appropriate.
The outer game
Prepare to talk with her at a time when neither of you is upset. Ask permission to give feedback first; if she refuses, try to find a different time. If she still declines, it'll be time to escalate.
Regardless of whether you talk with your colleague or her boss, you'll take a similar approach. Be specific, using "I" statements to get your point across. Instead of, "you have such a bad attitude sometimes," try, "When you ignore my urgent requests I get very stressed because I might miss my deadlines." Then ask an open-ended question, trying to understand what's causing it. Then do some joint problem-solving to see if there are different ways you can communicate with her that would be more effective.