Logic Pro 9 modulation in exs24


i am having some modulation problems on a track i am working on. i am trying to make a single bass note suddenly do a filter cutoff wobble and also spontaneously change the speed of the wobble.
i've set one of the lfo's to modulate the filter cutoff and used the sample & hold waveform on a different lfo to modulate the speed of the first lfo. the problem however is how to get the bass to only wobble some of the time, so that, at times, there's no wobble, but only a constant tone playing.
is there anyway to do this using modulation? the only way i know how to do this is using automation (lfo bypass on/off), but then that does not make it spontaneous.