How to stop erratic FPS jumps and tearing in Tekken 8 with DDR5 6400?

The experience was terrible; erratic jumps between 100 and 140 FPS caused severe horizontal tearing. I tried enabling V-Sync in the driver panel, but input lag spiked over 38ms per RTSS, making the game feel like walking through mud. I used the GamePP DLSS/FSR Image Sampling Calibrator to lock the sampling rate at 100% and enabled Enhanced Sync. Frame time converged to 6-8ms per RTSS, and the motherboard stayed at 56-62℃ per HWMonitor.