USB 2.0 device not recognized or not started or working fine

Hello everybody,
I am using a video grabber from Trident connected via USB 2.0 with Windows XP.
The behavior is really weird:
- sometimes not present in the device list
- sometimes not recognized (unknown device)
- sometimes recognized but not started (Code 10)
- sometimes working fine
I tried to use a sniffer to monitor messages over the USB.
This is what I discovered: the driver usbehci seems not able to open the port when something is going wrong.
I attach the trace taken with USBlyzer (and HMTL export files even if not detailed)
The issue is that after the Set Port Feature command, I don't see in the next Get Port Status the flag Port is Enabled.
I have several other traces, but I wouldn't like to write a message too long.

Thank you in advance to anybody willing to help me.
Sincerely

Andrea