MOTM

Hi,
I'm having a problem with one of my 300's : it occassionaly drifted a 
lot and it was working fine before.

I had noticed that putting some pressure on certain locations on the 
pcb could make it stable again. So I was hoping for a bad connection 
somewhere. I used my multimeter to check the connections that I thought 
had an influence on the oscillators pitch, but could not find anything 
strange.
Then I started measuring voltages and compare them to one of my 300's 
that is working OK. There is a lot of fluctuation in the voltage 
controlling the pitchof the broken 300, whilst (as to be expected) on 
my other 300 this has a stable voltage.
I kept measuring and I think I traced the error to the part of the 
circuit that is generating the VX and VREF voltage.(circuit diagram 1 
of 3) :
When I disconnect L3 and D3 from the TL074, the voltage output on pin 1 
of the TL074 is still fluctuating. 
I have checked the voltage supply to the TL074 and the ones to resistor 
R17 and R16 and they appear to be stable. 
When I remove the feedback resistor R18, the fluctuation stops on the 
output of the TL074.
So I believe that the TL074 could be the troublemaker, but how could I 
easily check this without having to replace this part first ? 
Could a failed TL072 cause such behaviour ?

Thanks in advance for any help !
steven