safari not connecting on imac, macbook pro

We are running Safari (7.0.3) on our iMac and are sporadically getting messages saying Safari "cannot connect to the server". Also getting the same problem (again sporadically) on the Macbook Pro (Safari v.5 something), and also on our old iPad. Have been using Chrome as an alternative, but still having some problems, although not as much. If Chrome can't connect to a page, I just try again and that seems to do the trick. However, this is getting really frustrating - seems to have just happened out of the blue and has been going on for the last couple of weeks. Any suggestions? Have reset the modem, disconnected etc to no avail. Thanks for any advice.