When on an ILS approach, when you're established on the localizer, the glideslope should take you at the right angle down to the airport on the VASI's. If 160 mean 160 knots you're way too slow for flaps 15 (or any flaps when not taking off) you should approach at about 180-190, and not get as low as V1 until within the decision height. You're too slow and the plane can't maintain altitude are the appropiate downward verticle speed because it can't obtain suffecient lift, when power is applied, the V/S goes more negative because the addition of power changes the pitch of the aircraft at the low speed, and you fall faster until dspeed is regained and it can keep the proper attitude.