I just tried installing the ACA620 in my other (non-working) A600, and mechanically it feels a lot more solid when installed in it. It seems to touch the motherboard all the way around and there is more of a 'locking' feeling when pressing it down (and no squeaking noises). The markings of the CPU are the same so I'm not sure why this could be.
I also tried booting the A600 without the ACA620 installed but with Acatune in startup-sequence and the program crashes. If I boot the Amiga without startup-sequence and simply type 'Acatune' it says 'Card detected: ACA630-25MHz' so obviously it will crash if it tries change something..
Edit: Jens, I tried your twist-test and it revealed a perfectly stable stock A600 capable of running memory tests while I was twisting the motherboard quite a bit (with no bottom shield). Combined with the mechanical feeling described above I'm still thinking the bad connection is between the CPU and the socket as it doesn't feel completely right. I am not sure why that is or what to do about it though. That being said, I will probably still resolder all the CPU pins for safety's sake if I haven't found another solution by this weekend. As you said, the socket pushes on the pins which could make a gap in a bad solder.
Last edited by demolition; 17 October 2012 at 22:09.