Decided to move my project source onto GitHub, so it is now publicly available for viewing:
After messing around with a new project, updating camera system and throwing in some improved synchronization of strings in the GUI, I realized that my little Estimator-class which handles all inter- and extrapolation was not working quite as intended.
The estimator classes are planned to have a cyclic array of states, where each state contains a vector (e.g. a position) and a timestamp. The thing with cyclic arrays is that when you reach the end, you are supposed to just restart from index 0. I had stupidly enough forgotten to mark a little boolean required for this to take effect properly (there were some other issues too, like which index to start at). Anyway, with that fixed both the static updates and Interpolation are now rid of the random off-frame issue which was one things I noted in the last playtest, but at that time attributed it to the testing software.
The extrapolator, however, requires even more work. I had been naive enough to think that just estimating a position would be enough. Or, well, I had some kind of interpolation/smoothing between the estimated values. The only issue was that the smoothing was poorly constructed and not working in the slightest bit as intended… Currently trying to add another interpolator for the estimated values, which I think should solve the problem…