First off, download the most recent version of the codec pack you use - I recommend either the K-Lite or CCCP pack, personally - and then UNinstall each and every codec pack you can find on your PC, before installing the new codec pack. ALWAYS uninstall any previous codec packs before installing a new one, even if it's from the same group!
If the problem persists after a fresh install, try the following (any order will do):
- Install your codec pack using ffdshow instead of the LAV filters, or vice versa, depending on which you use now.
- Use a different video and/or subtitle renderer in MPC-HC. Try to avoid using MPC's internal renderer, if possible.
- Turn OFF "Automatic exclusive mode during fullscreen" in LAV, if it's active. I've found it can lag my PC badly at times, especially during animated karaoke, for example.
- Update your video driver. Just like codecs, it's generally best to fully UNinstall any previous driver before installing a new one, by the way, even if "dirty" installs are allowed.
- Make sure your desktop resolution is the same as the native resolution of your monitor.
If none of that works, it will help for you to answer a few questions:
- Do ALL videos have this problem, or just certain ones? If only some files, do they share any specific characteristics (say, 10-bit, certain type of container - e.g. .avi or .mkv - video codec, or whatever; click on File>Properties for this info in either player)?
- What versions of Windows, VLC, and MPC-HC have you been using?
- When was the last time you scanned for malware/"greyware" (adware, spyware, and such)?