Packard Bell 60's Stereo- FM Issues

[Artcurus]

Hey All,
I have a Packard Bell early 60's tube stereo with FM Stereo/mpx (9TU tuner). The stereo has been rebuilt and is played 6-8 hours a day. But it has a developing problem that steadily getting worse.

At the 6-8 hour mark, the entire middle of the FM band drops out. The stations that are there are replaced by what sounds (very low volume) a V8 engine idling.

It also has a tendency to drift on the upper end of the band, 101 will end closer to 102.

The former tech replaced a bad 6HS6 with a 6AU6, which is an acceptable replacement but with lower gain and sensitivity. I live at the edge of a fringe reception area to complicate matters.

Any ideas?