Does it do the same thing with batteries and an external power supply?
When you say it 'turns horrible' and has a 'disgusting wall of mess' can you be any more specific?
If you turn it off, leave it for a while and then turn it on again, is the time it takes to go weird again directly proportional to how long its been left turned off?
If it is, it'd probably indicate that something is malfunctioning as it warms up and gets to a certain temperature. This could be something in the power supply section of the board, which might be easy to fix but difficult to track down, or the CEM3340 VCO chip, which could be easy to track down but expensive to fix. A load of new sliders etc from technology transplant is never going to fix either of these issues.
The first thing i'd probably try is to see if anything is getting hot. Specifically anything over on the far right of the synth board (with the back off, face down with the keys towards you) in the power area, or the CEM3340 (around the middle of the board, slightly off to the left)