That sounds strange.
When you say the results are unsatisfactory printing from other PC's is that with the correct drivers installed for your printer ?
The fact it's OK as a standalone unit kind of rules out print head issues etc.
Wonder if there is a "factory reset" that can be applied somehow ? Epson technical support I have found to be excellent in the past... well worth contacting I would say.
Have you tried unplugging the printer from the mains for a few hours and then seeing what happens.