Here are the specs:
-Strong X-90 projector
-Gate tension cranked to anywhere from 3 to 5 o'clock position
-Rails were just replaced in all gates not more than a couple months ago.
-No deposits on the rails
-No burrs or nicks on the sprockets
-Rails have been cleaned with rubbing alcohol
-No perf damage on any of the prints

If so, vertical jitter can be caused by using too much FG. It will jitter for a second and then stop. A few minutes later it will repeat, at seemingly random places throughout the film.

If this is a the case, then the solution would be to stop using FG on that print. The problem will subside after a while when the layer of liquid on the film evens out. If the problem is bad then you can load up a set of DRY media pads on your film cleaner and run the film through that. This will sop up any extra liquid from the film.

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Take a close look at the runners (sometimes called "straps") in the Trap. Make sure the surfaces are parallel. I've seen them installed where they are canted and it does nasty things to the picture.

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We don't run cleaners with FilmGuard on them like many people here do, we simply wipe it on the top of the film. Lately, we've been having insane amounts of looping, causing three brain wraps in a weeks time...when we haven't had any incident since September. So that means the Film Guard has been used more than normal.

I just thought it odd that it would happen in certain parts of the film. And yes, there is a very loud rattle when this happens.

The skis/traps/shoepads/straps (call em what you will) were replaced with great care. I even checked that twice before coming here.

But I'm led to believe that FilmGuard is the culprit since the projectors its been happening to have been the ones with significant looping problems.

We apply FilmGuard with a lint free cloth on the top of the print. We have been using more than the recommended ounce because of brain wraps at the worst times. We had one on a sold out Friday show and one on a sold out Saturday show. What we ended up doing was just having a projectionist/floor person come up and watch the prints while the other one threaded. (We're an 8 screen). Something we've never had to do before but a neccesity.

Our humidity levels have been hovering between 20 and 40%. But all winter, no matter what the humidifiers kicked out, we had on average a 20% reading with absolutely no looping problems.

You said earlier: "Lately, we've been having insane amounts of looping (interlocking?), causing three brain wraps in a weeks time...when we haven't had any incident since September."

Could it be that running the same print through several projectors is drying it out to the extent it is more prone to "static cling"? As the film exits the last projector, does it lie nearly flat, or is there significant "curl" or cupping from excessive dryness?

Look at the factors that influence "static cling": proper humidity, conductive platter and roller surfaces, winding orientation, winding tension, platter timing, restraining the roll to prevent "fling", etc.:
Platter Patter article

In severe cases of static cling, treatment of the full width of the film with an antistat is more effective than just the edges (i.e., use a slightly dampened media cleaner rather than wiping it on the edge of the film).

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion