The Mysterious disappearing printers

I need your expertise some of my users from credit dept are having trouble printing while the following application is open, SFD, Email, Infinium, i-many, ANSSWER, and POD Viewer.
The computer is Compaq pentium III 733 with 128 RAM.
Printer installed HP Deskjet 895, IBM4317, and HP laserjet 8150 network printer.

The problem is whenever they try to print, a message comes up that there is no printer available or the printer is not present. The printers mysteriously disappear from the Printers folder.

I tried reinstalling the printer driver, removing the LPT1 port, and it work after restarting the computer. But it keep doing the same thing after a while. I ‘m not sure if this a memory issue, programs or the printer itself. Any idea?