Mitsubishi CT-20AM1S(A) fault
 

I have a Mitsubishi CT-20AM1S(A) colour TV. The VH high band (Channels 7, 9,
10) wonders. You can tune it in and within a few days, it is off channel
again. If I run it through the video, it is fine.

I had taken it to the shop, but it came back the same.

Would it just be the tuner faulty or a little deeper than that?
Any help appreciated.