Network file transfers drop to zero after 1 hour

Working on a problem where large file transfers over a 1Gbps dedicated WAN circuit that runs over an MPLS cloud will run at optimal speed for an hour and throttle back to nearly zero. Occasionally the the file transfer will ramp up briefly then drop back down to a trickle of data flowing to the receiving end. Both ends are running RHEL5.5 on Dell Servers. We've checked that all ports are set to 1Gbps or greater from end to end. No packet loss, no retransmissions. Latency is 60ms end to end as the circuit is 2000 miles long.

This problem does not occur on UDP, only TCP. MTU is set to 1500 and the TCP stack on RHEL kernels is set to the default values.

net.ipv4.udp_wmem_min = 4096
net.ipv4.udp_rmem_min = 4096
net.ipv4.udp_mem = 4651200 6201600 9302400
net.ipv4.tcp_rmem = 4096 87380 16777216
net.ipv4.tcp_wmem = 4096 16384 16777216
net.ipv4.tcp_mem = 196608 262144 393216
net.ipv4.igmp_max_memberships = 20
net.core.optmem_max = 20480
net.core.rmem_default = 129024
net.core.wmem_default = 129024
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
vm.lowmem_reserve_ratio = 256 256 32
vm.overcommit_memory = 2