If it were me, I’d do the following:
1. Unplug, check and consider replacing the coax cable (wall to Horizon)
2. Ditto the HDMI cable (Horizon to TV)
3. Call UPC support and explain the issue.
I had a similar issue with my Horizon box and they sent an engineer. He ended up tightening up some loose connections in the wall socket and also replacing my coax cable. He had some fancy signal monitoring equipment to do the diagnostics.
Sorted my issue out.
I’m pretty sure the Horizon box will be supported for ages to come, as there are many people who still have one! The UPC TV box is a worthwhile upgrade but yes, the stanard packages cost around Fr.10 more. However, you get 7 day replay on all the UK channels whereas I believe the Horizon doesn’t.