Keyboard intermittently stops working

I am stumped to fix a problem that is affecting a PC used at a business.  The PC is a 3 year old Dell Optiplex 7010 running Windows 7 Pro SP1 (fully patched/updated).  Intermittently (somedays it stops several times a day, other days it works fine all day) keyboard stops functioning.  Keyboard has been replaced at least twice. Original keyboard is USB, but a PS/2 was substituted and in each case we saw the same pattern of failure.  USB mouse was replaced with a new one.  USB Power Management on all internal hubs is off.  USB keyboard and mouse are only USB peripherals in use.  The only solution we have is to reboot or power-cycle the PC.  This fixes the problem immediately, but a time later it returns.
We do have success with bringing up the on-screen keyboard, but the business cannot function this way.  Have not tried RDP access as the user does not have time to stop, call for help (contracted IT Support) and wait for a response.  Problem has gone on for over a month, so our monthly image would be useless to try.  I am the IT Support professional, have 21 years in the rear-view mirror and am at a loss where to go to help my client.  Any suggestions that I can try?