Shifting retention time in SIM analysis - 8890 GC/5977B MSD

I have been trying to develop a GC/MS Direct Inject SIM method and think I have landed on something that will work.  However, the retention time keeps shifting with each injection.  I am wondering if I need a longer equilibration time in between injections?  What might cause something like this?  The image below shows the same vial injected 3 times in a row.  My compound is eluting at ~4.77 minutes.  I used blue arrows to point out how every peak in the injection is shifting by the same amount. For example, the distance between the green peak @ 4.780 and the green peak @4.92 are the same as the corresponding peak in the black injection.  Thanks in advance for any ideas on what I might be able to change to get a more consistent RT.

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