That temperature (in the low seventies Centigrade) is within the Athlon's working range which goes up to a maximum of 90C (194F) but is a bit high for comfort and improved cooling would be a good idea. Getting the temperature down to the 120 to 140F region would be sufficient.
There seems to be something wrong with that modem though, since refusal to work when the CPU temperature is above 100F (37C) is very odd. It's impossible to say what that fault might be though since it could range from a dry joint to an internal chip fault. Assuming it's a PCI modem, it would be cheap and easy to replace though.