Contour cuts have chatter, rough change in direction, and random misalignments

Hello!
First may I put a disclaimer here that I'm new here today after having visited a number of times in the past for various issues. I think this is the right sub-forum to post this (holy jesus there are so many), but if not please let me know and I can relocate. I've tried searching for an existing thread with the issue I'm having without luck so I wanted to post a fresh topic with some specific details.

To start out, we use an older Techno CNC LC Series 4896 machine, paired with MasterCAM 2021 Industrial version. Our jobs are mainly 3/4" plywood contour cuts and pockets of various designs, a handful of small jobs throughout the year. I have a basic understanding of the machine and MasterCAM, but nothing too technical.

On to the problem.

Over the past couple years, I've noticed a slow but noticeable degradation in contour cut quality. Sinuous lines or circles became less precise and no longer match the input geometry as closely as designed. See cnc1.jpg for a recent example of this, taken between tool changes. The circle budges at the transition of the Y to X axis movement (moving counterclockwise). There's also a slight indent inwards where the bit begins/ends a pass. This occurred for the full set of 9 circles cut. I'm aware of the tuning of feed and speed to reduce chatter and have had positive results in the past, but this job was turned down to half the typical speed we'd run (in an effort to avoid these issues) and it still produced these abnormalities.

I'm not sure if it's caused from vibrations or axis problems or a combination of both - or something else entirely.

A second, more strange issue is a misalignment in passes that occurs along the Y axis only. It appears that very first pass of a contour is shifted just over 1/32" towards the Y origin. The second pass (same toolpath)then corrects itself back down. See cnc2.jpg. This has affected overall dimensional precision of pieces only along straight Y axis cuts.

One last problem that occurs in tandem to this, and I'm unable to explain as well, is a sudden jump back at the last 1-2 passes, resulting in a tiny shelf that' we've had to shave down. See cnc3.jpg. Occurs only along straight Y axis cuts as well. Thinking it might be linked to the problem above?

Apologies for the long form of this post and for any glaring omissions/stupidities on my end. I'm happy to provide more info too.

Thanks!