As for the PSU itself being a possible cause of tuning problems, I also doubt that too. Switching PSU, like the one used in the Voyagers, are by design tightly regulated/filtered. But that doesn't mean that they can't fail, or intermittently send high frequency noise on the power rails if they're badly designed. That wouldn't affect tuning, but could explain some other failures.
As I said before, having seen my fair share of failing cheaply designed/implemented switching PSU (mainly in inexpensive devices like DVD players and such), resulting in either strange/unexplained behavior or simply failure of other components due to "noisy" power rails, I'd rather trust a good 'Ol linear type PSU over switching type any day. Especially for analog synths.
The main enemy of electrolytic caps is heat and ripples, and there's plenty of that in a switching PSU.
This is the PSU in a Voyager. Often, you'll see caps next to a heat sink (as seen here) because of how cramped components usually are on such designs. Those caps will fail prematurely, guaranteed. It's only a matter of time.
Moog Voyager PSU
And, yes, the power connector on the main analog board (seen here) could have a poor connection resulting in unstable or under voltage power rail for the oscillators, that could affect tuning.
Voyager analog board
