Moog Source: CPU clock trouble

I have a Moog Source in for repair at the moment. It’s exhibiting all the classic ‘crazy source’ symptoms (stuck on patch 1 etc) but a previous repair has soldered the 5V regulators in place, thus negating the usual fix. Looking at the CPU, I noticed that the 2MHz clock signal is very unstable; lots of jitter. Not only that, but on each clock pulse the ringing imposed on the 5V supply rail is so severe that it dips below 4.25V, which is below the CPU’s minimum supply limit. See attached image; CPU clock input is the top trace; 5V rail at the CPU supply pin is the bottom trace, with 1V/div vertical scale. Pretty unpleasant!

To me this looked like a classic case of insufficient decoupling (and there isn’t much of that on the Source’s digital board). So I fitted a 100n cap across each IC and a 22u on the 5V where it enters the board. To my surprise this made no measurable difference at all to the ringing on the 5V rail. So my thoughts are now focussed on the clock circuit itself. I figure to replace the clock components, or alternatively to replace the existing discrete oscillator circuit with an encapsulated 4MHz clock oscillator device that feeds U22B directly. I’d be interested to know whether anyone else encountered this behaviour? A damaged crystal might explain the jitter, but assumedly not the supply rail ringing?