I have a friend who cuts me out every so often...do you think this is a valuable friendship or not?

I have a friend, when we I see her or talk on the phone we get along just fine and she always points out how similar we are in so many ways. She is also a supportive friend and seems to understand me more than many other people do. However i've been noticing at some point after a few months she will suddenly cut me out and not answer her phone or call me. She'll text me and say how she is busy. She's done this in the past a few times and after a long time when i do get in touch she says she's just been bored and not really doing anything. I don't think she's a busy as she makes out to the extent she can't even talk on the phone when I just needed a bit of advice from her. I think she has a problem but I think she's also being stubborn and selfish.
Even if it's the night before my exam and she calls me i'll speak to her and listen to her problem, yet when it's the other way round, she won't speak to me for weeks on end. Another thing I find annoying is that because she is away studying at a distant uni, when her mother needs me to do her a favour she calls me, and i'll be going out of my way taking a couple of hours or so out of my time to help her. I've also helped her little brother and cousins with their homework.
Do you think this is a true friendship? As sometimes it feels like an insult, as if she's bored of me and wants to distance off for a while. I feel like i'm giving more and she's calling the shots.