i'm posting this in the hope it will be of help to others, as the cause was something I hadn't encountered before.
After watching a perfect projection one night, the next night , when I ran a print it was very jumpy. As the print has been run several times before with no problem, and all my prints are well lubricated, my first thought was to check the pull down claws for any build up of dirt or fluff. The claws were clean. The next thing I did was to re-lubricate the print, as it was a few years since it was done, though I used a one off application. When I ran the print it was exactly the same as before. I have noticed for some time that if you turn the inching knob, it is quite stiff, plus the tension seemed uneven, it was stiff pulling the claws up, then seemed easy as the claws dropped down. So I took the back off, and very carefully applied sewing machine oil to all moving parts, wiping off any excess. Afterwards I noticed the inching knob turns much easier. I thought that maybe the uneven tension was causing the jumpy image. I ran the print again, and it was still the same, then I got out another film to try and that was just as bad. I was now running out of options. I searched out an old combined gate brush and scraper, and with the aid of a magnifier, I looked for any dirt build up in the where the gate and aperture are. I couldn't see anything. And then poking around, I found the brass film guide opposite the claws was movable by about 1/16th of an inch. I presume this is to accommodate any variation in the width of the film. When I pushed this back on it's spring I noticed a black line, and using the scraper, I found it was dirt, and the brass showed underneath. When I had removed all the dirt, and given it a final touch with the gate brush. I gave it a very quick burst of WD40. Since then I have shown about 5 feature films without any issue. I have at times moved the frame line so I could check for steadiness. The very small amount of dirt, must have been enough to prevent the proper movement of the guide, which made the tension on the film uneven, and the image jumpy. I looked at it again tonight,and there was a very small amount of dirt, so I have cleaned it again, and this time I used a small amount of machine oil, as WD40 is a dry lubricant. One other thing, the machine was quite noisy, as if you were running a dry print; after the cleaning it became much quieter.
When you think about it, whichever guage we are using, the image is magnified many hundreds of times, and it only requires something very minute to be noticeable on screen.

I have taken a photo and arrowed where the problem was, so I hope this is all made clear.

Good One Robert, at least it was an easy fix, (even though it was annoying to spot). My Elf had this problem after i took it out of hibernation of ten years. That movable side plate was jammed with dirt, then the tension spring broke so i used one from a spare one i had at the time. It doesn't take to much to upset our machines in both gauges.
Glad its all good now.

robert, thats one of the most neglected parts of the film gate, clean after every showing as you would everything else.
all of the hell and bowells i've collected over the years never had this part of the gate cleaned, i'd push it back with a stick and would be black
with debris.

Thank you very much for your advice Clyde; this will now be part of my regular cleaning routine.. I cleaned it again tonight as it was starting again, near the end of a film, though I need to check the splices , which I haven't done. I mentioned this to Hugh Scott, and in all his years, he never had this issue. My films have been well lubricated , and I'm wondering if that causes the problem because any dirt on the film is softened up.
Another strange thing, I don't know if it's connected with the earlier lubricating of the pulleys of cleaning the film gate, but the sound seems to have improved, I'm sure it's not imagination. I haven't touched the optical system.

16mm is very different in this area to Super 8mm machines.

All the Super 8mm machines I use just have short length sprung, curved guides which move in and out very very easily. These are usually at the top of the gate to encourage the film to dead centre beyond entry and the opposite way around at the bottom of the gate.
4 curved shaped pieces in total controlled by a hairpin type spring on one side and fixed on the opposite side to provide an accurate datum edge for the film to run along.
I've never noticed any particular amounts of dirt ever gathering in or around these areas on 8mm as most of the gate is uncovered by these guides.

There are gate brushes and there are gate brushes, but the best to use in this case is a toothbrush. As you slide it up and down apply gentle sideways pressures against the gate tension spring.
Clean after each show.

Just limit the use of Colgate on the brush though! 😂😂

Your gate will have a pearly white veneer in no time!

It's taken about 8 or 9 years for this to happen, but now I shall clean it regularly. What's the difference with gate brushes Maurice. The one I used was made by Arrow, and has a scraper at the other end.

I have many gate brushes but most are very similar, unknown makes. Mine which has a plastic scraper on the end does not have a brush section which is parallel. Therefore, as mentioned above, a toothbrush (with teeth) is much better for the job in hand.

Here is how you finish up looking if you share the same toothbrush as your projector gate uses!



Recognize this man anyone? Don't do it!!