Fingers That Slide Off The Black Keys

I have been playing the piano as an amateur for a long time (24 years), but have not taken lessons for over 15 years. I have at home an old East-German upright piano, a Geyer, which suits me as it does not have enough sound to entertain the whole building! I have noticed something which has been developing over time, that is, that when I hit a black note either I miss it completely or the finger slides from the black key, hitting the white one next to it. At times, when playing the last movement of Beethoven's sonata op 27/2 I find at times I just cannot hit the unison g#, the same going for Chopin's Etude (Revolutionary). I cannot figure out what causes this and how I could solve it.