Steve.

Possibly your new RJ-11 to USB adaptor has a device isolation or power regulation issue and is allowing an impermissible current flow or voltage spikes back to the host computer.

Remember that inside the OnLogic computer all the USB ports will be connected to only one or two motherboard USB chipsets and any current overload or over-voltage spikes on one of the physical USB sockets may propagate across other USB devices sharing the same chipset, causing them to disconnect before Windows has a chance to shut down the USB device causing the problem.

Some compact motherboards incorporate the USB chipset directly into the south-bridge chip which means that sporadic USB disturbances can be even more disruptive as other non-USB peripheral devices sharing the south-bridge may be affected too.

Where practical, use isolated USB to RS232 adaptors, or, include a USB-USB opto-isolator in the path between device and computer to help mitigate against such problems.

Was anything recorded in the Windows system logs with a time stamp that correlated with the disconnects?
There should have been something logged if Windows saw a USB device misbehaving or suddenly disappearing while actively transferring data.

William.