Okay, I was super busy with other things, but now finally found some time to work on this project again.
I’ve replaced the 3360 with a new AS3360, but the behaviour of the synth hasn’t changed one bit alas.
I’m not sure if the symptoms started happening at the same time - I basically got it in non working condition.
Okay, so it might be an idea to hunt down a microprocessor and hope that that’ll solve the problem?
I’m nw officially out of ideas and feel like this project is well over my head now. From now I’m just poking in the dark, so if anybody has ideas, I’d be very grateful!
I know I dind’t need to recap the PSU, but then again, while I was at it and the Synth is rather old, I thought it couldn’t hurt. Well, it didn’t, but it didn’t help either 
I’m very much aware of this and I’m super grateful for all your help thus far! Thank you!