Vintage Synth Repair

Ok, this is making things a bit clearer. I'm thinking about two scenarios here:

1. Q7 turns off a little too late. This allows C8 to discharge for a very
small time. Different possibilities:

1.1. Q7 itself is faulty. You said you already replaced it so this isn't
very likely but cannot be ruled out.
1.2. The various keyboard contacts break contact in the wrong order.
Usually the gate buss should break contact first, then the CV buss. Is the
behaviour uniform for all keys? Is the behaviour identical when releasing
the key slowly or quickly?
1.3. Other things in the path from the gate buss to Q7.


2. The circuit finds some kind of "balance" after Q7 turns off.

2.1. Are +/-15V exactly identical? Is there possibly ripple on one of the
rails?
2.2. Perhaps something's wrong with Q8.


At this point I would probably first check the power supply voltages and
the try to see (possibly with a two channel scope) if the busses are
switching in the right order and if there is some delayed switching of Q7
(compared to the actual gate buss).