proxy server

I”ve recently installed a MS-Proxy on a NT 4.0 server.I’ve a 64 kps ISDN Line for accesssing the Internet.The problems i’m facing are as below :
1.Even though the line is OK,the ISDN connection drops after 10-15 Min. all by itself.When i Try to redial gives Error:678
2.The ISDN line is 64 kbps but when i use it in even 5 PCs for internet, it becomes very slow..even slower than PSTN..

PL. give me the solution to the ab. mentioned problem.

Thank You