Hmm... One more piece of advice that generally works for me on such random-looking issues, then I'm out of ideas. Is the printer directly into a power outlet--not an extension cord, not even a power strip or surge protector. Dirty power can cause many, many things.
@mrfixitpc: the 5L and the 5Si aren't THAT different. The 5Si's most differing feature was the upgraded printer engine (and it does make a difference in actual printing), but the drivers interact with the printers in much the same way.
