Problem with Number changing, seemingly, on its own

[ABL]

I have a strange problem, and I'm starting to draw blood from beating my
head against the wall.
Basically, I have a table with both an autonumber field, customer_id,
(used for links to other tables) which is transparent to the user, and a
user id field, cnum, which the user may or may not change.

Cust_num is automatically generated at record creation. I count through
the records and find the first unused number:
custnum = 0
Do
custnum = custnum + 1
Loop Until DCount("*", "customer", "[cnum] = " & custnum) = 0

Perhaps, not the most elegant, but it gets the job done in another form,
I'm using.

Basically, the user enters a new customer name (they can also select an
existing one, but that functionality works okay) and cnum is generated.
I am not using a bound field, so I set Me!cust_num = custnum, after
record creation, before I leave my customername_NotInList sub. After
some other assignments, I have:
Me!cust_num.setfocus

So far, so good.

I have set Debug.Print points at the end of NotInList, and created subs
for cust_num SetFocus, GotFocus, BeforeUpdate and AfterUpdate. So the
order of items and the characteristics are:
cust_num.GotFocus (correct answer)
NotInList (correct answer)
Cust_num.beforeupdate (WRONG answer = customer_id)
cust_num.LostFocus (WRONG answer = customer_id)

I have this SAME code in a different form, and it works. Maybe I was
just lucky with the other form, but I'm lost for a solution.

Has anyone seen anything like this? What am I not telling you that
might help?

I am using Windows XP and Access 2K SP3 (MS VB 6.3). A push in the right
direction would be appreciated.

Thanks,
Alden

PLEASE do NOT reply to this, but post to the newsgroup. Inbound email
is not checked.