Multiple FD40 PRI failover switch failure

Hello, we are experiencing sporadic failover switch failures at a production site. The symptoms are: Our ISDN PRI lines are disconnected, and indicator LEDs on failover switches go out when that happens. Reinit of failover switches always rectifies the situation (somebody on site unplugs and plugs the USB connectors for a second), that is why we suspect, that the failover switches are the problem. There are 4 failover switches at the site.
When this happens, there are no indications of failover switch activity anywhere in system logs, nor do PRI lines fail over. There are no USB logs in system logs. Can anybody please help us with this situation, are there any known issues with 4 FD40 failover switches running simultaneously on a PBX? Considering the fact, that power indicator LEDs go out, could it be a power issue? Could USB power supply be overwhelmed by the power requirements of 4 FD40 switches maintaining combined 16 PRI lines?

System details:
4 OpenVox FD40 failover switches carrying 16 ISDN PRI lines
DELL PowerEdge T710 (Intel ICH9 chipset with Intel 82801I USB controler)
Red Hat linux 5.5 running stock kernel 2.6.18-194.11.4.el5 SMP
Asterisk 1.2.16
res_failover.c lists no version, but is dated 2008 OpenVox inc

Failover device configuration:
[openvox_failover_1]
device=/dev/ttyUSB0 (the only difference between devices is in device file)
kick_time_interval=5000
event_cmd=no
event_cmd=logger
autorun=yes