Error Code sc 899 mpc 4501

I can't seem to get rid of sc 899 on a MPC 4501.

I updated all firmware and formated the HD.
Checked all the computers printdrivers (all MACs) Postscript option installed.
The unit works fine throughout the day. After it goes into sleep mode for some time approxamitly 15-20 minutes the sc899 error pops up.
After that occurs we can restart the unit and works fine (print, copy, fax, scan) everything works fine until it goes to sleep and then the cycle starts again.
Anybody see this and perhaps resolve this most annoying problem.

Thanks
George