Help with Polar 720 power option

On the past two rides, my beloved Polar 720 power option stopped recording power properly while recording speed and cadence properly. It happened suddenly midway during a ride when the power values reported dropped on average to only 30% of the actual value. It also report pedaling index as around 70% (usually 25-30%). It has done this ever since. Since I have a 2nd complete power unit I was able to trade components and was able to identify the magnet unit as the main problem. My S-720 unit works fine with the the 2nd unit. I mailed it to Polar for servicing and they send it back withou being able to find anything wrong. I reinstalled it and things were fine for 20 miles after which the same problem recurred. Does anyone have any clue what's going on and what I could try? Thanks