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Unusual Rank Problem...

Here's a stumper for you: 
     Our Mark III is currently exhibiting a small problem that has even 
stumped the folks at Rank.  It seems that we have an unstable tach pulse 
output from our sepmag driver.  The unstable pulse creates a flutter 
effect on our venerable MagnaTech follower. 
     Touching the film just before it enters the wheel indicates that 
the film itself is vibrating!  This vibration is then translated through 
the tach board resulting in an audible flutter.  When the MagnaTech is 
not running in interlock, it is very stable. 
     It is worth noting that this vibration is only apparent on the 
stretch of film from the silver precision roller (or the 35mm roller  
below it) running up to the wheel.  The entire left side of the 
transport is quiet, and the film is steady in the gate. 
     We've tried to replace the tach wheel (two separate replacements), 
the capstan, the rollers, and checked the supply and take-up tensions. 
The Rank organisation appears to be baffled. 
     Has anyone here ever come across anything similar?   
     Fortunately, in these modern times we don't have a lot of call for 
sepmag.  Unfortunately, we do have some call for it.  If I hold the film  
by its edges as it enters the tach wheel I can dampen the vibration 
somewhat but that's not a pleasant alternative to a real fix. 
     Hoping the vast collective experience of this group can proffer 
some suggestions...(sorry, we can't swing an Ursa just yet!) 
Modern Video 
Phila, PA