If your not a VM cable customer, then your probably using BT Openreach connection and they do have a habit of slowly decreasing a connection if they find a stability issue. Only problem is, they don't tell you or your ISP, so your pay full price for a crippled system.
If you are a VM cable customer or the above BT connection, get on to support and find out why.
Of course, you could have faulty hardware that does degrade as it heats up, but rare.