Experiencing Intermittent Dropouts

Since switching our computers to a Comcast connection, we have experienced intermittent dropouts, usually averaging 1-3 times a day and remaining out for an average of 3-5 minutes.  After having technicians out and calling and being told that there was nothing wrong, we went over our network setup with a fine toothed comb.  Nothing has made a difference in the amount of outages.  Last week I set up a PING both on our network, and to the Internet, and the results show that the network is staying up, but for my 3-5 minutes there is no internet connectivity.  Some help on this would be wonderful both for me and my frustrated patrons.