Can't be solely the fault of MyTether. I have in the past had MyTether on my Pre, but not for a while...then the 1.3.1 update happened, which is when I lost the ability to play almost all of the videos that were already on the phone (and which used to work before). I did have a bunch of patches that were on there before the update and that I didn't have a chance to remove before my Pre updated itself.

Then...thinking to fix it all (esp. those patches that I didn't remove before the update) and start from scratch, I WebOS Doctor'ed it, never installed MyTether, and then slowly re-installed a bunch of patches. Even before installing patches, with the clean, just-WebOS Doctor'ed phone, those problematic videos wouldn't play.

It's not just size, either, as I have smaller videos that don't play, and I have a 7xx MB converted-via-Handbrake movie that does play...