Loss of picture quality or recurrent reception issues can be caused by a variety of things but they'll all be related to the way digital TV is set up in your home. Things like misalignment of the UHF aerial (even by a few degrees) or satellite dish, loose or degraded connections, lack of the latest firmware update for your digital receiver etc. can all result in recurrent reception issues.
It's difficult to diagnose the cause without seeing the actual set-up, and even then it will involve a bit of problem-solving detective work.
Time for a little bit of investigating, and the trick is - be thorough.
It is best to check each component in your set-up, starting with the easiest such as connections. Try the troubleshooting tips on the Freeview website as a guide; if nothing there works and you've queried both the manufacturer and retailer of your digital TV equipment, then get a quote for an installer to come around and diagnose the issue - the installers will have all the gadgetry required to check all parts of your set-up and be able to recommend a solution.