A fine wine has to mature. It’s the same with classical instruments. Those early polyphonic synths tend to develop their own little quirks after many years of faithful service. You can look at it as a character quality! The Prophet-5, for instance, is characterized by increasing vagueness in certain electronic parts.
This means, for instance, that the oscillators go out of tune easily. But (!) it’s exactly this quality of the Prophet which lends it its remarkable expressive vigor. Which is why in the following (first) soundfile you hear an out-of-tune Prophet-5. Each voice has developed its own individual beating.
Can anything be more beautiful?