Intermittent performance issues with Solaris 10 NFS V3 client to RHEL 5.5 NFS server

I recently attached a Solaris 10 8/07 client (6900 with ce gigbit interface) to our NFS server which runs RHEL 5.5 (kernel 2.6.18-194.el5). Performance typically is good running around 25-50MB/s but sometimes seemingly without reason the performance drops to abysmal levels and will stay like that until NFS is unmounted and remounted. I have tested this after hours when there is no load on either server, no traffic on the network and using a 1GB test file. Our other 300 Linux clients have no performance issues, I have ruled out network issues by isolating the server to a switch dedicated to it and an additional port on the NFS server and the tests I performed were with the file cache in memory. 

$ time cp /var/tmp/1g.TEST.new /mnt/ 
real    25m1.456s
user    0m0.276s
sys     0m6.699s

After an unmount, wait 5 minutes and remount:

$ time cp /var/tmp/1g.TEST.new /mnt/ 
real    0m26.767s
user    0m0.277s
sys     0m6.589s

Mount options I am using on Solaris:

  Flags:         vers=3,proto=tcp,sec=none,hard,intr,link,symlink,acl,rsize=32768,wsize=32768,retrans=5,timeo=600
 Attr cache:    acregmin=120,acregmax=120,acdirmin=120,acdirmax=120