Did you consider the video driver?
I've consistently encountered this same slow motion problem with playback of recordings in progress with two PCs using three different drivers:
* Intel HD4600 (IGP of i5 4690K CPU): MC v20.0.12, Win7x64
* Nvidia's driver for GTX-780 graphics card (same i5 system above): MC v20.0.12, Win7x64
* Intel HD Graphics (IGP of Celeron G1840): MC v20.0.14, Win8.1x64
Once recordings have ended they all play back fine.
I use a dual-tuner SiliconDust HDHR3 source and if I select a live program different from the one being recorded it plays fine.
I also checked system resource utilization during slow-mo playback on the Intel Celeron system (the lower powered of my two systems) via Task Manager and no component (CPU, RAM, etc) was being stressed at all. Further, I've done playback of recordings in progress via WMC in the recent past on Gen3, Intel Celeron-based systems without incident so I don't believe system resource consumption is the issue.
I use the feature of playback of recordings in progress all the time with TV, so if the slow motion problem can't be fixed soon I'll have to move to another solution for TV recording and playback. Beyond the "showstopper" slow motion issue, I really miss several features of WMC such as, but not limited to, built in pseudo commercial skip, superior subscription setup via TV guide toggle of individual program, and better guide navigation). In summary, slow motion issue notwithstanding, MC works for TV recording and playback but not nearly as easily or effectively as WMC. FWIW, I've tried a few PC-base DVR solutions and none of them have matched WMC so far in terms of ease of use and logical function: Not to mention encrypted cable card support.
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