cant Access internet through 450G FOr some customers

i have problem with 450G . I have taken 10 mb bandwidth from isp .Using 450G as server Restricting Bandwidth through Queues . i have problem with some customers which are not accessing internet through 450g when ever i repair customer LAN card
then they will access internet for a while after that the same problem continues i must repair LAN to access net to that customer . And i have an another isp which is giving ip base .when i put another isp ip which not coming through 450G their is no problem with that . i cant understand this problem pls help me

Thanks in advance

Sounds like routing problem. When klient have the problem again, see if you can trace route from client router board to your server and the other way round.