Finally got to the bottom of it.
Reflowed the vco section twice and still intermittent. I noticed that if I removed the 4066 whilst a pattern was playing, the accent would stop entirely. Then I'd insert the ic back into the socket and it would work again. Come back 5 minutes later and it wasn't working properly again.
So I removed the socket and soldered the chip directly to the board, and it works every time now! I knew I hadn't done anything wrong, just a dodgy socket.
Thanks for all your help!