Peter, all Soviet radios with a turret wavechange arrangement tend to develop bad contacts with age, and it's very important to clean the contacts carefully, but I suspect Miguel is aware of that.
It's possible that the MW padder capacitor has failed open circuit, which would explain the frequency displacement.
The environmental conditions in Cuba are obviously very different to those in western Europe (and Canada
), so components will degrade in different ways and our experience here may not transfer well to Miguel's circumstances.
I would not expect a transistor fault to cause a change in the local oscillator frequency.