unstable Internet connection

scenario is that often we loose connection to internet the modem and router has to be restarted to re-establish connection to internet ,, I just had the modem replace 2 day and still have the same problem, well when I do down load +-100.00mb file, defiantly disconnects ,, do u think this has anything to do with ISP or our router because the LAN is quite stable no problems and what I have notice often is that when the connection dies I try to ping the router there is no response and I try to ping our Internet IP Address using a different ISP connected to other router no response too.