Some modems work better cold than hot, and vice versa.
Generally speaking it is a good idea to make sure that the modem does not get too hot. If you have your modem connected to the phone line through a surge suppresser, try it without the surge suppresser. Many surge suppressers can interfere with modem communications. If your connections are better, you have a premises problem that you may be able to isolate and fix. If that doesn't help, listen carefully to the quality of your voice connections.