First check the age of the strip, 880-100, the date is on the envelope or you can find a 3 digit code in the upper right hand corner of the strip (y, m, d). The strip should be changed about every 18 months.
Then check the cal strip to see if there are any wrinkles or creases that could keep it from reading properly. Replace if needed. If it looks ok then check at the lamp strength (+93%) and motor speed (1.0-1.4, should be the same or consecutive numbers).
Also blow the unit out with air to remove any contamination to the optics and try calibration again. If problem continues the unit will need to be serviced.