The next step could be to try it with other TVs. It happens that sometimes they don't auto-adjust to the slightly different syncs of the EP.

Check the 5V voltages not just at the regulators, check at chips, for example: U24, U25, U33 on +5A, ROM, onboard RAM, Z80 on a +5B
Nick chip use +5A, if it is lower can also result this wobbling effect. Also check +12V, for example on U35 Pin14,15,16.
And do you have a scope? Will be good check the Nick clock.

One more hint: If it is built with Philips capacitors then recommended to replace all.

Check the 5V voltages not just at the regulators, check at chips, for example: U24, U25, U33 on +5A, ROM, onboard RAM, Z80 on a +5B
Nick chip use +5A, if it is lower can also result this wobbling effect. Also check +12V, for example on U35 Pin14,15,16.
And do you have a scope? Will be good check the Nick clock.

Sorry, for  the months delay -  a few unforeseen events happened and I got distracted.

These are the voltages I measured between the  VCC and GND pins of the following chips:
U24 4.94
U25 4.94
U33 4.93
U35 11.87
RAM 4.92
ROM 4.91
Z80 4.91
NICK 4.92

I'm having trouble with my scope so I haven't managed to get the Nick chip clock yet - still working on that.

In order to help me identify the chips and  pinouts on the circuit board I made the following image - I'm attaching here in case someone else might find it useful. The XCF file is the GIMP original with the layers preserved for ease of editing.
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