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Børre Ludvigsen
That is my experience too. The pressure needed to engage the filmholder varies. If you look at my picture above (12-17-2010) of the cogwheels of the transport mechanism that pulls the filmholder in, you will see the half-circle metal plate that trips the light switch that starts the motor that pulls the filmholder. It all depends on the position of that plate - which is turned by the engagement of the inner cogwheel. If the plate has a long way to go (180°) while still between the light and sensor, you have to push a while (and the resistance can feel strong), before the switch is tripped. Worse still, if the tooth on the rack hits one on the cog directly, it's almost impossible to get it going without some "coaxing" back and forth. Not a very good solution. I also suspect that it's this process that might cause the cogwheel's axle to lift out of its housing with time.
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Børre Ludvigsen
The focus banding problem is solved. Use VueScan - Set "Input | Media" to "B/W negative" and set "Input | Make gray from" to "Green". The results are excellent! The reason is here https://www.dantestella.com/technical/scan612.html: ".... using the green channel for b/w means that only one CCD line will be used, making the scan go 3x faster and eliminating the "focus banding" you sometimes see with the Sprintscan and other scanners (this is related in part to "stitching" that occurs when the scanner goes back and forth to position each of the R, G and B lines on the same position). In addition, green should be the sharpest color for non-APO optics used in scanners. If scanners are in fact apochromatic, it doesn't matter which color you use."
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Børre Ludvigsen
HEADS UP!!! The "out of focus banding" problem is solved! Scan (B&W) from only one channel, for example green. That's it, perfect scans.