The stereo decoder in the MR71 and MR67 (found in the slightly raised section on the front-left side of the chassis) is a bit prone to drift, and IIRC the symptoms you describe are exactly that of when the 38KC oscillator transformer is out of whack. But unless you're vary [sic] familiar with what you're doing, I would be VERY careful twiddling with it . . . a quarter-turn of the top core of this transformer is the most I would attempt without test-gear. Also keep in mind that the slugs in these transformers are very brittle, and they can crack very easily with an aggressive hand on a metal screwdriver.
You might also check the condition or substitute the tubes in the stereo decoder, otherwise get the unit professionally serviced and aligned. In servicing a vintage FM tuner, it takes a bit of experience to understand what types of adjustment are required just to age and drift, and what kinds of symptoms indicate that something else is going bad. There's also a mod available to the output section to provide a real separation trimpot adjustment, which can increase the separation performance by a good 5-6dB over the original.
You might also check the condition or substitute the tubes in the stereo decoder, otherwise get the unit professionally serviced and aligned. In servicing a vintage FM tuner, it takes a bit of experience to understand what types of adjustment are required just to age and drift, and what kinds of symptoms indicate that something else is going bad. There's also a mod available to the output section to provide a real separation trimpot adjustment, which can increase the separation performance by a good 5-6dB over the original.