Elektor Formant Modular

Hi. After much fiddling around i got both of my formant VCO's to be very stable - hooray. - so i'm moving on to the 12 dB filter.
I get hi/lo/bandpass with various stages of succes..
I soldered in trimpots for r23, r27 and r6 so i could adjust everything instead of relying on resistor that had a non difined value. But for the life of me i cant get everything to work right.
-The LP works fine and i can get it to close all the way down/open fully. The HP i cant get to close completely. I always have a bit of HF coming through. Bandpass seems to work fine.
-A general problem with all the filter types:
If i leave freq in the middle and turn the q to max and adjust the trimpots so i get maximum q value, i get a nice resonance that sound great. Then if i turn the freq up i all of the sudden get a HUGE jump in q volume and the filter starts self oscillating, but only at the extreme high freq setting - if the q was consistent all over the freq. spectrum i could deal with it, but it is not...
Any suggestions? It seems to me that it could be an issue with a cap somewhere, but which one(s)? It would make sense to me that it could be C2 since it's close to IC1 which controls all of the various filter types.

Help much appreciated!