IRS-TR 11001: Temporal Responsivity Variations on the Red Peak-Up   Sub-Array
  Over the course of the cryogenic mission of the Spitzer Space Telescope, the responsivity of the Red Peak-Up sub-array on the Infrared Spectograph (IRS) varied by ~2%, based on an analysis of five standard stars. The sensitivity dropped 1.7% after the first 14 IRS campaigns, then climbed back up 1.0% later in the mission. The uncertainty in these measurements is better than ~0.3%. The random variations in the Peak-Up photometry of the standard stars has a gaussian distribution of width ~2%, similar to the magnitude of the systematic temporal variations. 
G.C. Sloan and D.A. Ludovici
astro-ph.IM