Can't Add New Records During Office Hours

I have a SQL 7.0 backend for a new DB of mine. This is the first time I
have developed a SQL backend from scratch, so the problem may be my design.
The problem may lie with my Access frontend (I have tried both 97 and 2k),
but I am inclined to think not considering the behavior.
On only one table (the master table no less), I cannot add records during
business hours. I can add records to other tables and edit records on this
table though. This troublesome table has a field indexed as unique that is
not the PK (don't know if that has any bearing, but it is the only table in
this DB that has that property).
The adding records problem appears to be network traffic related because
sometimes records can be added at lunchtime and definitely after hours. I currently only have 3 connected users. I had these problems with only one connected user, so the DB itself is not being strained by traffic. I often have more traffic on the DB after hours to avoid the problems. I have worked on the connection and network traffic angle for awhile, when I stumbled upon the bizarre part. If I add a record via a stored procedure and run this SP on the SQL query analyzer on the server (no network is involved) it is still slow to add the record (30 minutes or more). Needless to say this would equate to an failure on the front end. The same query will run in 0-3 seconds after hours!
Does anyone have any clue how this could happen? I am stumped.
I do have a budget for a consultant if I need one, but I need to find
someone who has a chance to fix this problem. If you don't know the answer
but can refer someone in the DFW area, I am in your debt.

Debbie