Being a multi-user environment, the first question would be is it split into a Front End/Back End configuration, with the Back End (with all Tables) on a shared drive and a copy of the Front End (with everything except the Tables) on each user's hard drive?
If it is not split, you have to understand that having multiple users sharing a single, non-split database, sitting on a network drive is the sure way to repeated episodes of corruption, speed and timing problems, and all manner of strange, odd and curious behavior, including the problem you're having, now! Multiple users simply have to work off of a split database, with each user having their own copy of the Front End, which contains everything except the data/tables, on their respective hard drives, and a Back End with only the Tables on a shared drive.
Being in forced retirement, I spend 8-10 hours a day here and on other Access forums/newsgroups, and over the past seven years have seen dozens and dozens of reports of non-split apps causing these kinds of problems as well as massive data lose! The really insidious thing is that a non-split app can work for extended periods of time before the troubles begin! But once it does, they become chronic, which is to say they occur over and over and over again!
The record, by the way, was a Non-Split db that had been working, without problems, for 12 years! It then started exhibiting all of the symptoms listed above and continued to do so until the app was split.
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007