Is a certain modem (COM port) freezing, or could it be any of the 4 modems (COM ports)?
After this issue happens - but before you power cycle the Rapidport/4 - is the Rapidport/4 still listed in Windows Device Manager? Look in the Modems category to see if the 4 modems are still there.
What software are you using to dial out?
Can you reproduce this issue using other software, for example forcing the modem to dial out using HyperTerminal, PuTTY.exe, etc.?