W2k RRAS problems

I installed w2k on a proliant p380 g2 with w2k server. Then installed 3 Digi Accelport Ras modem cards. Everything worked fine for 1 month, Then suddenly i started to recieve 721 errors on dailup. I have replaced the DIGI cards, the pci cage , and the motherboard. I have rebuild the server and i still have this problem. I then hooked up an external moden to com 1 and was able to connect. What have i missed
Jeff