Sporadic printing and scanning freeze

I recently upgraded to a Mac mini (Intel) with OS 10.8.3 (Mountain Lion). I have a fairly old Epson Stylus Photo R340 printer (still working well) and a newish Epson Perfection V500 Photo scanner, both of which worked reliably with my old PPC (Tiger) computer. When I upgraded to my new computer, I downloaded and installed the latest drivers for both the printer and the scanner.

I now find that sometimes when I want to print something (generally after doing other work on the computer and browsing the Net), the print dialog appears, but after clicking 'Print', the printer does not do anything. If I then shut down and restart the computer, the printer immediately — even before I have signed in with my password for the computer — starts printing the document or web page that I had previously selected.

This proves that the material was transmitted to and stored in the printer's memory in the first place, but the printer could not be activated.

What persuades me that this is a computer system problem and not a printer malfunction is that invariably when this problem arises with printing, a related problem simultaneously occurs with the scanner: if I try to scan something, an error message appears, saying that no scanner is connected to the computer. And restarting the computer clears that problem, too.

I should be grateful for any suggestions that might help me avoid this time-wasting problem.