Technological reason for being 'jerky'?

Many scenes have short visual "hitches", as though a handful of frames were missing, maybe because they were cut out. or maybe because the camera mechanism stuck momemtarily (or maybe because they didn't survive age and restoration). These hitches occur very roughly something like three per minute on average, although their distribution is very wide, so sometimes they occur much more often than that and other times much less. They seem to happen more often on cold outdoor people scenes, although to some extent they affect every kind of footage (outdoor warm, outdoor cold, indoor, moving people, static landscapes, etc.). Some are barely noticeable while others are rather jarring.

I associate the overall impression with many films of that era. Lots of other people do too; a common complaint about silent films is many of them feel "jerky".

I assume this is something about technology ...but I can't figure out what exactly. Anybody know what was going on?