Duplicate Autonumbers Being Generated

I have an orders database using Ac2K. Main tables are:
tblOrders: Has OrderID which is autonumber, key index (no duplicates)

tblOrderDetail: Has OrderDetailID --long integer

Tables are related in one to many relationships on OrderID-OrderDetailID with both cascades set.

Database is split with tables and front end on server. I can't put front end on desktops because users can access from any of over 600 computers in the school district even tho there are not all that many simultaneous users.

Data is input on main form bound to tblOrders. When user puts in required information (Date,Vendor other etc.), I save record and autonumber generates a new OrderID. The subform window then opens and user then inputs the details of the order to the tblOrderDetail. There can be multiple entries here.

Form and subform are linked by their ID fields as master-child.

PROBLEM: System which runs on network with 2-3 simultaneous users is unstable. It works for a while (maybe 10-20 orders) and then crashes. The subform window is blank (never opens).

When I examine the tables directly I found these problems:

1) Two of the OrderID's (autonumbers) are the SAME! They are usually right above and below each other and usually tied to the same user altho other order info may be different. I don't see any data "collision" if it's the same user. I thought this was IMPOSSIBLE.

2) There is an entry or two in the tblOrderDetail with ######### in one or more fields. The OrderDetailID field is blank. I once found the ######### in the tblOrders as well.

3) The key index is removed from the OrderID.

4) When I go to the relationships window either the link is missing or the cascade and integrity are no longer active. If I try to re-establish, I get a message about missing key index or unmatched data. If I then go and DELETE the records with the ##########, I can re-establish the relationship. And it works for awhile again.

5. Compact and repair doesn't fix anything.

6. One user "thinks" she saw a quick message about "time-out". Is there some setting in Access that I need to change (Tools-Options-Advanced?)

ANY ideas would be much appreciated. There is nothing worse than trying to work with a system that works most, but not all, of the time.