[Arran]

I wonder if the IF is at a lower frequency and you are aligning it to a harmonic? Another possibility may be bad resistors, though you likely already too care of that, or it's possible that a fixed mica in the front end has decided to cause problems due to age? I would not beat your head against the wall on this, it's a cheap set so it it picked up whatever was local the original owner would have been happy.
Regards
Arran