I am teaching myself to play the violin and have been the last few years and I think I have made fairly good progress yet I still find it difficult to get clean notes at all time. It occurs often on the the A string but is not exclusive to that string. When it happens I can feel it in my fingers as I press down on the strings. It has a certain vibration and the sound kind of prolongs and then cracks and go over in another note like when someone yodels or sing country music. I have done research about it and have been practicing bowing as accurate as possible between the bridge and fingerboard, I have tried with less and more rosin and varied the pressure and bow strokes, yet I am not sure how to improve it. I bought the Concert Violin from the Fiddler shop a few years ago. Could it be the quality of the bow or the violin?
My flute vendor recommended an annual service (I'll take that with a pinch of salt, as I don't think I ever got my oboe serviced, lol, and a friend's wife is a flute technician anyway, and I plan to learn how from her when I get around to it). Violins should probably be serviced every few years, especially if you are self-taught and don't have a teacher to spot problems with the instrument and you have set some things up yourself and done things like change strings without supervision, maybe shifting the bridge.
Like Andrew says, change strings every 6-12 months. I've made strings last 12 months, but it's silly. They start unwinding before the sound changes. That's bad, as you can injure a finger on an unwinding string, and, worse, if you injure your finger unexpectedly, you may flinch and throw the violin across the room, so I'm planning to change strings every 6 months, no matter what.
So, to summarise, if you live close to Fiddlershop and the shipping is cheap, and Fiddlershop will service instruments, that may be a good thing to do. It may be the only way to get an accurate diagnosis.
If you find C# is the worst note, you may find some of your afterlengths are tuned to C#. That happened to me. A quick fix was to put mutes on the offending afterlengths to change their frequency, but you learn to compensate for things like that after a while with your bowing, and so it may not have been necessary. (you get similar things on other instruments - C an octave above middle C is very weak on the oboe, and beginners assume that they have to buy a better oboe, but when your breath control is better it becomes a perfectly good note)
And that brings us to the question of how much you have been able to teach yourself in the time you have been learning. It may just be that you need more experience, more bow confidence.