...didja' ever go to start your car and it won't...since you unplugged the dvd player(was it just the player OR the interconnects AND the player?)that pretty much rules out a problem with your pre, more or less(see below)...could be a failing cable, a problem with the player or it could have been there all along, but wasn't noticed until you "clued" into it.

You really shouldn't connect or disconnect anything with the power on, it's not good a practice...WAY down isn't off.

Intermittent problems are a technicians nightmare...without seeing a schematic I can't be sure, but there may be a terminal strip grounding problem in it's nascent stages...either that or your connectors may simply need a cleaning with an appropriate solution or the every-so-often pull-and-twist mechanical scrubbing(WITH THE POWER OFF!!!)...I've found the change of the seasons to cause some contacts to get cruddy...

jimHJJ(...good luck...)