As i said previously - disconnect entire CS. Use only LCD and check whats happening. If no problems with stucking into ENV page then definitely you have something wrong with CS. Another thing is to check CS connector there may be weak connection. Next shorts!! Check every switch. Im not sayin that any of them is broken but it helps you localise which is which. ENV page if i remember - second is responsible or somehow. Next you may disassemble ICs from CS and mount only shift registers responsible for buttons, try different order. Blinking leds are last thing what you need now;)
Turns out to have been a few issues compounding into one wonky Sammich.
Bank sticks issue - I’m fairly sure this was PSU related. I’ve moved to a 2A 12v switching supply that I used for a Tripath amp previously. I’ll look for a better quality linear (or build my own).
CS and LED issues - putting this down to poor solder work. When I built this box 3 years ago, it was about my 3rd ever proper project and I was using lead free process with a 300 degree C tip and cheapo Chinese fake flux. I ended up using a heat gun to pull the resistor network below the first CS register, cleaned up the pads and resoldered. After this work the bottom left switch was acting up causing all kinds of CS weirdness (gently tapping it made the issue occur).
Heat gun removed and resoldered the switch.
Reflowed most of the board with the heat gun and my soldering iron (mine is now a toasted Sammich )
After that - Sammich played nice for the rest of the evening - both in Sidplayer mode and synthesis.
Thanks for all your help and moral support. I can now haz Sammich.
)