Thanks for you patience.
I tried to replicate this on two different Windows 11 computers and I am not sure I am seeing what you saw. I did a lot of timing experiments and never saw variance in the 0.5 second range, I only saw a difference of about 1-2 milliseconds between frames. Nevertheless I did update the way the program calculates FPS in an effort to workaround this issue. You will find the new binaries in the pacman-rc.zip. I hope they solve the problem.