Since purchasing this instrument used in 2019 we have been experiencing the issue of shifting masses/AMUs. So far the main board, side board and power control board have all been replaced. A new diffusion pump was installed the last time it was serviced in April 2021. We have also installed new columns and electron multipliers. The MSD currently has no issues passing autotune, standard spectra tune of a BFB target tune. Shortly after however the AMUs start to shift by more than 0.1. The higher masses are affected most and will shift so much that peaks are not even present when doing a profile scan in manual tune. This MSD is in tandem with a 6890 GC and is used for volatiles analysis using an EST Evolution concentrator and Centurion autosampler. This issue occurs no matter what filament is in use or the polarity of the detector. We are out of ideas on how to troubleshoot or fix the shifting AMU issue on this MSD. Any ideas or thoughts would be greatly appreciated.