Vintage Synth Repair

I'm handling a friend's mini, which acts strangely. The overall pitch (of the entire machine) will change suddenly, up and down, not to far from the correct pitch but a few semitones... It affects all oscillators at the same time. Is this a known issue in old minimoogs? I would usually suspect either power supply main voltage instability, bad caps or transistors, but being it affecting all 3 oscillators, I tend to think it is not oscillator circuit related ? :/ I'm new to minimoogs so any help would be appreciated... Thanks !