Network Problems

I have three sites connected through frame-relay links and two of these
sites are quite fine in terms of performance. The third site has a weird
problem. The link is there 95% of the time. Testing using ping to check link
availability. However despite the fact that the link always says its there.
If I try to access the computers at this site you wait for a very long time
before you see somethng coming thru. command net view on windows computer
will yield results after about 3-5 minutes most of the time. During this
period you will not get timeout on the link. 


site A----------------64K F/Relay------------------------Site B----------64K
F/Relay---------------- Site C

Site A to B OK
Site A to C Chaos

How can solve this mystery. Ping works fine between Site A and C but sending
data across the wont bring joy