[Yaesu] FT-897 audio bug

[Tom WA9WSJ]

Hello Gents,
Has anyone a problem with their FT-897 transmit audio intermittently fading slightly in the FM mode?
I noticed this on a friends signal tonight. With very short transmissions it doesn't appear but longer, like close to a minute or more, it fades and recovers, then fades, recovers fades again etc..never just stops.
Using a stock MH-31 mic.
Any thoughts about this?
thanks,
Tom