Remember that even 56k modems still can only 'send' data at 33.6k max . In general, internal modems that are installed as an adapter 'card' into an ISA or PCI slot are 'hard' modems. That is they have their own DSP chip onboard the same as external modems and function equally as well as external types. They have the added advantage of not needing an external power supply or desk space. Software conflicts can seriously affect the stability of 'soft' modems.
External modems can become quite hot during their normal operations.
However, if they overheat, they may start to perform erratically. To combat this, place your modem away from your PC, preferably in a well-ventilated area. Some home and business alarm systems have a back to base feature, which allows the alarm to be remotely monitored by a security service. Such services make use of your telephone line and may interfere with your modem. If this is the case, try using another telephone line or consult your security service for possible solutions.