Fluttering notes at a bridge is a simple issue: more air is being
removed from the player action than is being admitted into it at the
point that the bridge covers the tracker bar.  That indicates porous
pouches or over-size bleeds, both of which remove more air from behind
the pouch than was intended by its design.  If it's only on a few rolls
then the system is simply in a marginal condition.  Some modern-era
rolls do have bridges that no player was ever designed to cope with,
though.

Old pouches get less airtight with time, and new pouches are often of a
different porosity to originals.  Making the pouches of an airtightness
matching the original design is the best approach, re-sizing (reducing)
the bleeds is in my book less desirable because it's changing a fixed
design parameter of the system.  MMS has loads of discussion on sealing
pouches.  I use Hydrophane or neatsfoot oil, which make the leather's
fibres swell hence reducing its porosity.  Things that stiffen the
leather have bad effects on the player and so are to be avoided.

Julian Dyer