EN CV Mixer Problem

[A.S.P.]

For the last three or four days I've been trying to finish up a pair of 
Electronotes CV mixers from the Musical Engineers Handbook (Sec. 5D 
P.7).  Everything works fine, except when I connect the output to an amp, 
I get a very high frequency signal mixed into output (around 600 KHz, 
5VPP).  

I built the circuit exactly like the diagram plus a few additions - caps 
to filter PS noise, a 30 PF cap across the output buffer, a level pot, 
and a second output. I've gone over the circuit with a fine tooth comb 3 
or 4 times and found no mistakes.  Now I'm wondering if it's possible 
that there might be a problem with the design itself.  

Is there anyone on the list that has this circuit diagram that would look 
at it for me, just so I know whether that is the problem or not?  Any 
other advice would be appreciated as well.

ROmeo