Things to check for:
1) Printers vanishing is oftentimes an indication of a bad print driver, meaning it is crashing the Printer spooler, making the printer vanish. Check the services log and see messages concerning the printers.
2) Insure print spooling is enabled in the print driver for all printers. Microsoft does not recommend turning print spooling off, especially for networked printers. See the following document http://technet.microsoft.com/en-us/library/cc739118(v=ws.10).aspx
3) Some Print Driver manufacturers install a proprietary print processor, you can switch to them use the standard windows server Winprint print processor instead. This can be checked via the Print processor button on the Advanced Print Properties tab.
3) If the printer is HP compatible install the HP universal print driver
