For the WF to look correct in the vertical ( and for WSJT-X to work correctly ) the A-D converter ( sound card device ) must be at set to 48kbps ( 16 bits ) , the data must be presented to WSJT-X uninterrupted and the CPU must have enough available cycles to process and display the data uninterrupted.  LatencyMon would tell you if you PC is up to the task.  Or you might have an unstable sound card device. It might be worthwhile to try a different sound card device to try and isolate the problem.
AL, K0VM