I'm not a particular fan of the parts placement in that layout. C8 should be closer to pin 1. I would put a 100nF cap in parallel with it, too.
Try lifting Pin 4 from ground, by bending the leg before placing it in the IC socket. I've had problems with it working like this, but other people report a slight improvement in noise when you allow digital ground to flow through the internal 10 ohm resistor between pin 3 and 4.
a 100nF cap in parallel with C11 wouldn't hurt either.
is it in an enclosure already? once it's in a grounded enclosure, that will help a lot.
maybe shielded wire would help, too.