two broadband internet connections on same computer

short answer no

long answer the way dsl works is that you send a username and password typically to verify and then you are provided with an ip to use two ips would mean to steal internet unless your paying for two connections. If you don't think your old serverice performed as well as your new one you should take it up with your dsl company. What it looks like to me is that your new modem doesn't work as well as your old one. You can go out and buy a new one or yet again complain to your dsl company.