New Share not reliable

My client is having difficulties sharing files between two computers. The
‘server’ PC is running Vista Ultimate SP1 and the ‘client’ is running Vista
Business SP1. Access to files in a subfolder under the built-in Users share
seem normal. However, when I setup a new folder directly off the C:\ drive
and shared it, files and folders are not reliably accessible from another PC.

Eventually, I want to restrict the new share to specific users, but
initially, I set it up where the Everyone group is a co-owner.

After the client computer starts up, it can access all files in the new
share normally. However, after about 30 minutes of use, file access slows
down. There is a delay of a few minutes sometimes before the close, delete or
modify task finishes. Sometimes, the client PC hangs indefinitely and must be
shut down.

I would appreciate suggestions to determine if this is a rights issue or a
networking issue and how I can find a solution.