asterisk users - Aug 2007 - Problems on SIP gateway (especially Planet VIP-450)

Hi all,
  I am having problem in using SIP gateway in a very high volume
environment (4+ concurrent dial-in) and phone calls in every seconds.
There are totally 12 phone lines. Currently I am using 6x VIP-450 for
12 FXS and 12 FXO. However, in this high volume environment, problems
encountered:

  1) After the gateway running for about 30 minutes, the VIP-450
becomes very unstable. When I try to dial from the FXS port to the
Linux Asterisk server, it jammed. Asterisk did not know what happened.

  2) Some gateways do connect to server, however no voice from the
asterisk's Playback function.

  3) Some gateways disappear in 'sip show peers' (Even if I do qualify
them). They forget to register every 300 seconds.. (that's stupid)

  I thought it was particular gateway problem, isn't it? VIP-450 was a
good gateway in low volume environment (10 calls per hour). If so, If
I changed everything to a TDM-400 cards, can the Digium TDM-400 afford
such a high volume and busy environment?

Yours sincerely,
Jason Chan, Hong Kong