Did they not give you a special filter to attach to the phone jack to plug the DSL modem into?
If they say they tested it, and no errors, in addition to having no problems with phone calls, I would suspect that the line itself is good.
If you suspect it is still your PC, try this.... Log into the router config page, CLEAR the log, the disconnect your PC from the router for a few hours. After whatever amount of time, connect back to the router, and check the log, see if it does the same thing with the computer unplugged.