As a ex Sky engineer i’d say the problem will either be with the dish/lnb or Cabling from the dish.
Its a time saving bodge by the engineer to swap the box out rather than finding the underlying problem.
Normally what happens is there is some signal related issue that causes the Set top box to work overtime correcting errors which it will only do for so long before it gives up the ghost.
New box gives the illusion its sorted until it too gives up the ghost with similar symptoms.
Wouldn’t surprise me if you have water in the cable from the dish shorting out the signal from one of the feeds.
What will happen there will be the good primary feed will allow you to watch live TV, the secondary feed with the bad signal will then fail any recordings your box tryst to make using it.
Only way to fix it is for the engineer to get busy doing proper fault finding replacing any damaged cables or lnb, checking dish alignment and for interference from outside sources such as alarm motion sensors, speed cameras, mobile phone masts and even aircraft!