  The SPIRE Fourier Transform Spectrometer on board the Herschel Space
Observatory had two standard spectral resolution modes for science
observations: high resolution (HR) and low resolution (LR), which could also be
performed in sequence (H+LR). A comparison of the HR and LR resolution spectra
taken in this sequential mode, revealed a systematic discrepancy in the
continuum level. Analysing the data at different stages during standard
pipeline processing, demonstrates the telescope and instrument emission affect
HR and H+LR observations in a systematically different way. The origin of this
difference is found to lie in the variation of both the telescope and
instrument response functions, while it is triggered by fast variation of the
instrument temperatures. As it is not possible to trace the evolution of the
response functions through auxiliary housekeeping parameters, the calibration
cannot be corrected analytically. Therefore an empirical correction for LR
spectra has been developed, which removes the systematic noise introduced by
the variation of the response functions.
