I assume you mean FORWARD the ports to your pc rather than open them (but anyway).
I dont think its the router as the port being forwarded is only realy relevent as the connection is created, after that its not the router forwarding but the alrerady existing link that is in use that controls where the packets go, forwarding only effects unsolicited traffic coming in on port X (if im wrong in a big way someone please correct).
I would however suggest you reduce the range of ports you use, as its possable this huge range can have unexpextent problems (i experenced this sometimes), try a range like 4001-4100 forwarded in the router to your pc, and a range set in mirc ALT-o/connect/options/advanced/dcc port first 4001 last 4099 (4099 becuase mirc seems to sometimes over shoot it by 1).
Even 100 ports is a huge range, so u could use 4001-4010 or something.
