Films with only a main stripe havnt caused me any problems in the past.
Some years ago, I had a problem with a number of films that were unsteady with up and down movement. While I'm no expert, movement in the gate, if it isn't the film itself, must be a problem within the gate/pressure plate. Or, I have been reliably informed, over cleaning can cause these sort of problem's as well.
In my case it was the main gate backplate assembly which, luckily I had replaced by Bill Parsons when he was in business so my Elmo was in the best possible hands.
One thing he did mention was that you can adjust these parts correctly, but if you have a particular film or two which always seem to want to be a little jumpy, when you adjust it for these films the majority of the collection will jump. In other words, the film is likely to be the cause in some cases.
I sent mine to Bill because my up and down movement started with one film which was my 400ft version of Bugsy Malone, it was fine on another projector, but again, as Bill said at the time, no two projectors (of the same model) are the same. After a while I noticed this unsteadiness began to happen to one or two other titles which I knew to be fine, I was never going to attempt playing with the gate and pressure pad, that would be the end of the projector if I even tried it. The adjustments are far too fine for anyone who doesn't knows what their doing to play with as I found out the hard way with one of my 16mm machines. The Elmo projector came back perfect as always. I was very lucky.
As for the balance stripe, this was meant to be for even focus so the film is level on both sides through the gate. To date, I have not had any problems with focus on any of our single track films although they are all flat, no scope prints, that may well make a difference on screen possibly.
BTW, I am not suggesting your problem is in the gate, merely pointing out that is was the cause in my case.