There's several different bad modes of failing/failed mica caps in the
IF xfmrs. As related to gain as long as the xfmr shows good peaks
during alignment the value should be correct. The thunderstorm noise
and intermittent operation usually occur before the thing goes
completely open.

Are you getting regeneration? I found I could not achieve
regeneration until everything was quite up to par. That seems like a
good measure of its overall health so if there are still problems with
the meter in the AM mode that might be localized to something more
specific than general IF gain.

-Bill