I've been digging into this hitching issue that some users are experiencing in the game. I now know for a fact that the old version of UE4 (4.18) works perfectly fine, and the instant I upgrade to the current version (4.19), hitching starts to happen. So something changed in the engine, and I have no idea what. Using the profiling/statistics features of the engine shows me that there is a specific function of the engine that is causing the problems, but it's nothing I have control over. And using Process Hacker shows that every hitch is caused by hundreds of thousands of page faults (where the game tries to access memory that it hasn't mapped yet). So it's something to do with memory management, but I'm re-using actors/assets everywhere I can be, and it wasn't a problem in 4.18...
So why don't I just downgrade from 4.19 to 4.18? Unfortunately, UE4 does not allow developers to downgrade engine versions. I do have an old copy of the project from just before I upgraded from 4.18 to 4.19, which is the game version 1.0.3. So for those of you who are having hitching issues, I've uploaded that old build on Steam under the "1.0.3 - Old version" build. If you go to the Betas tab of the game, you can opt into that. Notably, leaderboards will be missing from that version, but there shouldn't be any hitching problems. See the previous patch notes listing for what changed in 1.0.4 and 1.0.5 - those features will be missing from this build. That said, the 1.0.3 build is in shipping configuration for improved performance.
Going forward, I will resolve this issue with the latest build of the game one way or another. Of course I'm hoping a UE4 patch to 4.19 comes out that just resolves it (would be really nice since I'm still working 60hrs/week at my day job), but I'll continue to investigate every option available to me. Sorry for the inconvenience to those affected!