Video freezes for a few seconds in the begining

It looks like what @mike808 said, it’s due to keyframes at the cutpoints (or lack of) which is causing the tearing. This is an artifact from the original recording where it may not have enough keyframes.

There’s one way to try to workaround is to use the nearest keyframe which works in some cases (depending on how far it is). Try adding this to your profile:

CommercialMergeTool=avidemux

You can find more details about this option here: