Check the speed & duplex settings on both the server & the LAN switch. It sounds like there is a mismatch somewhere and this could be causing the problem you are seeing. Resetting the NIC would take care of this each time but you should hard set all server NICs and switch ports attached to servers.
This might sound obvious but you should check that only one of the network cards has a default gateway set. I had a similar problem with a windows server box running ISA. I had two gateways set and the server would run for several hours problem free then it would appear to loose connectivity.
chk-lan cable is fixed properly & if possible interchange lan cable b/w diff ports-disable & enable the lan & chk local area n/w is connected-still not-restart the system normally..