Drop in pressure slight vibration

Once again in need of all of your wisdom.

1975 Southwind 20, escort 351.
I was out this weekend, engine was running strong without issue. Saturday, I noticed that the oil pressure seemed a little low. By little, I mean just below the 40 mark at speed, but it usually is sitting right on the mark. I checked the oil, she was full, and since it was a slight decrease, I decided to just monitor. Sunday, when I went to pull her out of the water I was running about 2200rpm just in plane for about 20 min, when it felt like someone bumped the throttle down and started a noticable a vibration. we dropped out of plane without any change in throttle and the oil pressure dipped even lower. I dropped her to an idle, and she idled fine, but when getting back up to speed the vibration was still present and it was noticeably running with with less power.

I immediately put her on the trailer so I could investigate further.

Can someone offer me a place to start or even better, a list of of things to inspect that would cause something like this? Currently around 600 hours.

Like Pete sez.

next steps inspect plugs for any irregular, and do a compression survey