I have a very similar issue, for one I can´t even start the game over gog-galaxy. It just crashes instantly. I can however run it by using the crysis.exe.

If I do that I have the exact same stutter issue, I have 32GB RAM, an R7 1800x and Vega64. It runs with high FPS but stutters or even freezes for a second and longer every 20-30 seconds. This gets worse the longer the game runs and I noticed some weird RAM allocation:
Open Afterburner, monitor RAM usage. It starts with 3GB used in menu, then ingame it starts to climb until it reaches 4165MB. As soon as I hit that number it freezes, purges the RAM or sth. and drops back to 3GB usage. Then climbs again until it freezes and again and again. This process speeds up and after 10 minutes it instantly jumps from 3 to 4GB and down until it crashes.

Seems to be a critical memory leak of some sort and an issue with allocation size, like it does not use the RAM space I have.

I have not found a fix yet, currently trying to replace the 64bit binaries from a different source.

Does this game run fine for anyone with the gog version? Seems like it is pretty broken.