i water my oncidiums twice a week. most of my orchids actually. shriveling is usually lack of water. the mushy roots should be removed carefully, as they are probably rotting, which lets the plant get less water, and then it shrivels. normally i would say the plant is too dry, except the mushy part... are the bottom of the pots sitting in water? the humidity tray shoudl hold enough water to help give humidity, but the pots need to be above the water so they don't soak in it. do youhave the trays filled with gravel? if you post some photos, we might be able to help better. mushy is subjective, and not able to tell really, but a photo would help a ton.