Laptop restarts indefinitely on upgrading to Natty & using Huawei E173 modem

I use an Acer Aspire i5 machine and recently I upgraded my OS from Ubuntu 10.10 to Natty 11.04 Beta 1. When I connect my modem (USB type) it runs for some time and suddenly the system restarts leading to a loss of work that is going on. It happened for many times (3-4) within 1 hour use of System. I have seen this problem may be/ may not be due to E173 Huawei Modem. Also when the screen goes ideal, the machine restarts. Please Help.