For a month at seemingly random times while working with a project, playback in the main sequence and within the preview window is probably 20X normal speed. I can't tell you what I caused it, but it seems to be when switching between sequence tabs in the timeline area. When this HUGE jump in speed happens, the only solution is to restart Premiere Pro. Sometimes even doing THAT isn't adequate and I have to reboot my Mac. I have uninstalled PP and reinstalled it several times. I have reset preferences. These don't help any.
I'm using an iMac Pro (2017) with 64Gb of RAM, 3GHz Intel Zeon W, running Mojave 10.14.3. My Premiere Pro is the latest version from Creative Cloud.
I have wondered if this is a QuickTime problem because I've discovered that a video viewed directly in the QT app will occasionally do the same thing: SUPER high-speed, unusable video with no audio. My QuickTime version is 10.5 (928.50.1). I remember with Final Cut Pro how closely linked FCP was to the correct version of QT. I'm ready to get this behind me. Does anyone have any suggestions? I searched this forum and saw nothing. I don't see a QuickTime forum here.