Help me troubleshoot this problem

I have been experiencing a very odd random tonal change over the past 2 years on the analog side of my system.
The sound swings from glorious to mediocre with no apparent cause. or time frame. When it deteriorates, the change is always the same:
No bass below 50hz, narrowed soundstage, almost sounds mono, and elevated, somewhat strident treble. This change can occur during during play of one side of a record or when flipping over a record. It sometimes lasts weeks and then goes back the other way.

I have narrowed this down to something in the interface between the cartridge arm and table. All electronics have been rechecked by the makers and I have substituted different line, phono stages and amps and it still occurs. Also, the change impacts both channels at the same time and my line and phono stages have completely different left/right channels and I use monoblocks.

Unfortunately, I own just one phono cartridge at present so that part I can’t verify. I have checked vtf, vta, azmimuth and also had a local dealer who sets up turntables stop by with his rig and it is fine.

I use a line conditioner but using wall power dos not change anything, except it does not sound as good.
I could understand the system just not sounding good on an extended basis but the random switch to magnificent sound is baffling. I have verified this with REW; the loss of bass and exaggerated treble are measurable.
Anyone ever experience anything this odd? Equipment list follows.

• Garrad 301 (Xact Audio rebuild with plinth and outboard power supply
• Kuzma 4 point 14" arm.
• Koetsu Blue lace.
• Thomas Mayer 10Y line stage and D3A phono.Thomas Mayer 300B amps.
• Audio Kinesis LCS subwoofer with Ampino amp
• PAP Trio 15 speakers with Voxativ field coils, outboard crossovers.
• Elgar 6006B line conditioner

Thanks