There used to be a problem with the MXe that had a similar problem where the switch would come up and work fine for 20 minutes or so and then everything would lose IP connectivity. This was the L2 switch on the chassis faulty and the only cure was a new chassis. The work around to keep it working was;
"You can issue the command "period 900,mips_reset" without quotations into vxworks and this will reset the L2 switch every 900 secs"