6 out of my 7 network printers have this problem for the past 2 months.

Printer Agent Status Details: LPR Communication Failed.

Associated Message from Printer: Background test of printer connection failed, error: 36.

The only way to print after the message is to manually turn off and on the printers. I can ping all of them during the error.
I also manually installed one printer TCP/IP with the same result: "user intervention required".

After restarting the printers, user can print for 1 or 2 hours.
The error happen when someone send a print job.

What should I look for?
Thanks for your help.