I would suspect cold solder joint at frequency
Crystal or around L26 freq tx alignment point. Cold solder joints are basicly breaks in the solder or the 2 surfaces that were being solder were no equally heated when solder was applied. These tiny crack can expand with heat or develope corrosion that becomes an insulator. I suggest that you remelt the solder adding a rosin core silver solder in this area on trace side or have it done if you are not comfortable with soldering.
A good test is to remove speaker/component cover removed with radio upside down. Power up radio, antenna connected, mic gain down, key mic then lightly press down on the crystals there are 3 marked as X1 X2 and X4. Watch for devation on frequency counter.
It is unlikely that you have a bad crystal. I suspected voltage is dropped in this area it could be a capacitor. Since you mentioned Base i assume it is not mobile less likely it is cold solder joint.
Whatever the problem i suggest you go thru replacing capacitor it could make it much more stable.
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