@Matthew "Does anyone have any bright ideas on how to solve the intermittent motion problem without gutting/using old projectors?"
There is, quite understandably, a lot of misconception surrounding the subject of home-made gate/claw mechanisms. It is instinctive to think that extremely accurate and high tolerance engineering is necessary (especially for the minuscule 8mm format), but that is not actually so. If it was, low-cost 8mm movie cameras and projectors of yesteryear could not have been produced.
Years ago, I was involved in the design of 35mm animation cameras for use on multi-plane rostrums, but have not considered this technology for many years - until recently when I inherited a large collection of films made by my father and his uncle, dating back to the early 1900's. These comprise just about every format that exists up to 16mm, including the earliest that were hand-perforated.
I do not have projectors that will handle all these formats, and besides I know that many of the films are very fragile (not nitrate, fortunately) and may not survive the brutality of a projector. That is when I discovered the Kinograph website which I found very interesting. However, I can see that it needs a relatively expensive live-mode camera in order to capture the frame within a very few milliseconds, which I do not have. There are a few webcams out here that have full 1080 HD resolution (for stills), and excellent optics (which can be easily converted in seconds to macro), but their image chips are scanned (unlike DSLRs), which makes them unsuitable for moving subjects.
So, I revisited my past, and investigated if it was feasible to make a mechanism with my 3D printer (Ultimaker2) that would meet all my requirements. This time, I did a complete analysis of my earlier work, including a worst-case tolerance assessment. I was also able to adapt the design for 3D printing, which can produce parts that are difficult to make using traditional machining methods.
The result is illustrated in this animated image.
The dimensions are generic, and do not relate to any particular film gauge but, although some parts are quite small (especially for 8mm) they can easily be printed in ABS, nylon or carbon-fibre to the required tolerance.
For example, for Std8mm, the mechanism could have a pull-down of 4mm, with a claw-height of 0.85mm, given a tolerance of +/- 0.1mm. For larger gauges, this tolerance could be relaxed.
I have produced an Excel spreadsheet, which explains how claw mechanisms work, the relationship between pull-down and claw-height, and tables which show the trade-off between each of these parameters, for variable tolerances. There are pages that detail 9.5mm, Std8, Super8, and 16mm. I do not want to post that here, but if anyone has a genuine interest in it, then I am willing to email them a copy. My address is firstname.lastname@example.org.