Well i know theres a lot of things run on the 433mhz band Ie the rfid (nfc) , some wireless kit . Weather stations,  could go on all day.  As this is the older mk1 i suspect its interference from a mobile phone nfc or wireless scanning.   
  
Also on the 433mhz range (band) theres a limited amount of time you can broadcast a signal .   
  
So in a nut shell without knowing what kit the op is carrying phones tablets car keyfob ect.   
Where the gentleman is fishing . Ie near houses business park ect.   
Cant solve the problem.   
  
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