MD83 First Officer reports pressurization flow light at FL350 with the cabin climbing. A descent is initiated with ATC approval but the passenger oxygen masks deploy with the aircraft descending through 17;000 FT.

Narrative

At FL350 the First Officer noticed that the pressurization flow light had illuminated. The cabin was at 7;000 FT (normal) but the cabin vertical speed (V/S) was climbing at 600 FPM. The Captain informed ATC that the flight would need to descend. Center turned the flight 30 left and issued a descent to FL300 and soon to FL240. The First Officer pulled full spoilers and began the descent. When the thrust was pulled to idle; the cabin V/S showed a climb at the peg (at least 1;500 FPM). The First Officer added enough power to bring the cabin V/S to 1;100 FPM climb and still descend about 2;500 FPM. At FL260 the cabin stabilized for about 30 seconds and then began climbing again. The crew continued the descent and received clearance to lower altitude. At 17;000 FT the cabin altitude reached 15;000 FT and the passenger oxygen masks deployed. The crew continued the descent to 13;000 FT when the cabin stabilized and began to descend at about 300 FPM. The rest of the flight was relatively normal; considering the weather conditions.[It is] unknown why the depressurization occurred. Note: the previous day on this aircraft the FLOW light illuminated during the descent; but the cabin V/S showed a 300 FPM descent and the cabin altitude was decreasing. The FLOW light went out after a couple of minutes and the arrival was uneventful. The crew oxygen masks were used intermittently during the descent.