[R-390] 16khz filter failure when hot

[Jim Miller]

My 16 khz filter appears to "quit" when operating temperatures rise after an
hour or so of operation.  Carrier meter drops to near zero, although signals
can still be heard weakly in the 16 khz position (other positions are fine).
Use some component cooler to cool the 16k filter down, and it springs back
to life after a brief noise.  Is this a usual failure mode for this filter?
Anyone ever open one up to attempt repair?  Jim N4BE