Many thanks, bilou!. But as I mentioned, the problema also happens without the varicap. The fact is, I've never closed the loop yet... the freq jump would surely fool the stabiliser into thinking I am purposely turning the knob.
Maybe this phenomenom can be explained by some friend with close knowledge of audio preamplifiers? (because this must be a low freq noise modulating the oscillator). Meanwhile I'll try with other FETs and/or increasing the source resistor. I read dr. Ulrich Rohde advised not to use the gate clamping diode.