downloads stop at 40%

HI
just recently we replace our linksys router and our Cox Cable Modem. ever since, all large downloads above 20 MB will stop at about 40%.
the linksys support made some changes that lead to no where. Cox Comm. says it is not our problem.
anyone has any ideas ??
thank
Faris H. Haj
sys admin
pacific coast Biz. Magazine.