Improper charge for Trouble Call

Last month we were having intermittent outages, we called tech support. They were not able to assist over the phone. They sent a tech. He was here for several hours. He claimed he could find nothing wrong with the comcast equipment but the problem resolved while he was here and has not returned. We did not have anyone look at or fix anything on any other part of our network. They are now charging us $99.95 for the trouble call claiming that the problem was not comcast's.  We are a small business, we do not have a dedicated network administrator. What in the world should/could we have done differently? How can they be sure it was not their equipment when their tech was the only one who touched the system (and apparently fixed it) and their equipment was the only equipment touched when the problem was resolved?  We pay $6000 per year for this service and we cannot turn to them for assistance????  I can't get anyone on the phone to consider waiving the charge and the owner of the company is really frustrated and had me short pay the invoice. I know next week they'll be threatening service interruption. This feels like complete extortion. I would LOVE someone from Comcast to get back to me and help me with this problem. I could understand if the tech didn't fix it and we had to call someone else to come and do something else but that was not the case.