Have you had problems with your mouse before? It sounds like a hardware problem, most likely...

Check your kernel message log by opening a gnome-terminal session, then when the mouse disappears, type "dmesg | tail -30". This shows the last 30 lines of the kernel log.

Alternatively, look at the file /var/log/syslog after you've lost the mouse and brought it back. The kernel messages are logged there, along with other programs.

If you see something related to USB or /dev/mouse, or USB HID, or usbcore, feel free to copy *those* messages to this thread, and someone will try to help out!

Dave

In MEPIS 6.5, check the in the MEPIS X-Windows Assistant, Mouse tab, have you selected USB? It defaults to a PS2 mouse, selecting USB may help MEPIS keep track of your mouse.