network problems

hi
my company 30 sub office which connected to main office through shiva accessport.
main office= had router 500shiva two 24 port switches 24integrator one domain one server exchange and firewall as/400 running client express two metaframe.
suboffice= had connection isdn access port they running client express program which communicate with with main office as400.
we have problem start two weeks ago sub office disconnecting after every hour some time after 3hour if we run ping to leave the open line it working but when we stop ping it disconnected from sub office.