The frequency determining caps for each voice are often low drift types, such as polystyrene or silver mica. If you replace them with "std" mylar (polyester) caps, or worse yet ceramic ones, you will be degrading the stability of the circuit. If the tuning issue is common to all of the voices, you may want to look elsewhere, like in the circuit that measures the pitch during tuning. A stuck bit in that circuit would cause mis-tuning, I would think. It seems unlikely that all of the voices would have tuning problems at the same time. If the main crystal is off frequency, that would throw off the tuning, but that seems unlikely. Something to check for, of course. if you have an oscilloscope that can measure frequency.

Bob