Whoa! I am assuming that you have a digital readout instrument to measure the peak pressure, and that you are using the same on another cylinder where you have same readout at different different running condition.
First thing I would like to investigate is the instrument: I was accustomed by...
Maybe a little late, but it's my two cents contribution.
Rules first, as usual!
It is a requirement of Classification Societies ( ABS, Lloyd's Register of shipping, R.I.Na. and so on...), that an " OIL MIST DETECTOR" device shall be installed on every marine engine, used for propulsion purposes...