In my experience there is no such thing as a partial XLR connection that can repeatedly be dialed in at -5 or 6dB. It's usually either all on or all off.
The steady -6dB gives me another clue: one of the components may be touching ground, and decreasing output due to the inner isolation sleeve between circuit board and housing tube (ground) having been rubbed through.
Do this: next time it's low in output, use a couple of strong knuckle knocks up and down the mic housing, and observe whether the output changes. Also make sure that the -10dB stalk is fully in the low position (no attenuation.)
On the other hand, if this is one of those Klaus-guy modded 84's all bets are off... you may be a victim of that infamous German quality control!