Hi & welcome to TSF
I Presume that you have had the Aerial and coax feeder checked and found to be OK (I would think it is if you are getting a good solid analogue signal.)
It would appear that there is an intermittent fault in the TV - the hardest type of fault to track down!
However it is still under warranty therefore it is not recommended that try & fix it yourself.
Pursue the fault procedures with the place where you bought it from. Keep a detailed and clear record of all the faults.
Keep a history log of when it was returned for repair, how long it was away etc. This is the evidence of it's unreliability.
If you have enough proof of it being faulty from early days, *try* demanding a replacement TV because you have lost faith in it's reliability.
The fact that the 'engineers' have not found a fault/problem does not mean that there isn't one.