Large Pressure Drop 1100 Series GPC

I have been having some issues with pressure stability during injection cycles on our 1100 series GPC. Recently the purge valve frit and seal, and entire outlet check valve assembly from the isocratic pump have been replaced. Fresh columns with guard were newly installed alongside previously mentioned parts. While equilibrating the new columns pressure was stable, +/- 0.25bar, 1ml/min @ 66-67bar. Calibration standards along with a few other samples were ran with no significant fluctuations in pressure. Approximately at 10 injection cycles on the columns, large pressure drops began to appear. Pressure drop is of the same magnitude each time and cycles up to regular operating pressure in one minute. This drop in pressure does not occur at the same point within the injection cycle and appears to be random. 

The vertical red line marks the start of an injection cycle, I am also noticing a small drop in pressure at the start of a new cycle and am unsure if this is normal or if the needle seat assembly should be inspected. I am new to GPC/SEC, any help or tips are appreciated. Thank you.

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