Verizon isnt degrading your signal on purpose. To think that is just crazy.
There's a couple things that can cause a bad signal. One being a bad splitter. Bad splitters (I had one before) will cause issues.
This would be the first thing to check.
Other possible causes would be the line coming into your house or the ONT itself.
If your internet/router is working fine then I doubt its an ONT issue or a line issue.
Bad coax outlets/connections can cause issues as well, but from your description it sure sounds to me like a bad splitter.