lifecam vx 2000, Black screen, win7 64 bits

Hi,
I am going to be crazy. I have problems with my webcams VX-2000.
I am trying to connect 2 webcams VX-2000 simultaneously to a PC for video surveillance.
Sometimes, and I don't know why, the preview in Microsoft Lifecam software is blank (black).
The only way to resolve this problem is to disconnect and reconnect the Webcam that is malfunctioning and then the preview displays the real image.
And after many time (I don't know how many: it seems to be a random problem) the problem is coming back and I am far away from the PC: I am connecting to it with Logmein, and with Logmein it is impossible to disconnect and reconnect the Webcam.

I have already tried all what I found on the net about this problem:
Troubleshoot bad or no video or problems starting LifeCam software
LifeCam software displays an error or the video preview is blank.
Resolve compatibility issues between LifeCam and other programs
Problems with LifeCam VX 3000 software
Nothing works for me.

I have tried on many configurations and the results are:
- (Acer Aspire X3910) Pentium Dual Core E5800@3,20GHz + 4Go RAM + Win7 64 bits + 2 Lifecam VX 2000 simultaneously – The Problem is present
- (Acer Aspire X1920) Pentium Dual Core E6700@3,20GHz + 4Go RAM + Win7 64 bits + 2 Lifecam VX 2000 simultaneously – The Problem is present
- Pentium Dual Core E5500@2,80GHz + 2Go RAM + Win7 64 bits + 2 Lifecam VX 2000 simultaneously – The Problem seems to be not present

I have many theories about this problem, tell me what seems possible to you or what do you think about my problem:
1- The combinaison Win7 64 Bits + 4Go RAM + 2 Lifecam VX 2000 simultaneously is malfunctioning
2- Acer PCs have some running programs that are the reason of this problem
3- The USB Hub is not powered enough for 2 lifecams VX 2000 simultaneously
4- The USB Hub has not enough bandwith for 2 lifecams VX 2000 simultaneously

I used AutoScreenShotMaker software to capture the screen then view the captured. Hope it helps.