Hi Matt,
Several things that might help/shed light on things.
Bundled software does not always have the same capability as the full versions. If this has always been a problem that might be why - if it has worked in the past then it's probably something else.
Clear out any Temp folder and files
Make sure QT is updated.
Uninstall and then reinstall PD. It is sometimes necessary to correct any corrupt files.
Defrag your hard drive, especially if it's some time since you did so.
Use something like mediainfo
http://mediainfo.sourceforge.net/en
to check the details of your .mp4 files.
Let us know how you get on.
Cheers
Adrian
Life is really simple, but we insist on making it complicated. (see below)
Confucius
AMD Phenom IIX6 1055T, win10, 5 internal drives, 7 usb drives, struggling power supply.