Date Of Birth Conversion

Hello
Can someone help please?
I'm using SQL Server 7 and trying to update a table containing over 250,000 records. I have a character(8) (CHARDOB) field representing dates of birth in the form YYYYMMDD and would like to update my datetime field (DOB) using a simple update command like:-

UPDATE MyTable SET DOB = RIGHT(CHARDOB, 2) + '/' + SUBSTRING(CHARDOB,
5, 2) + '/' + LEFT(CHARDOB, 4)

but get the message:-

Server: Msg 242, Level 16, State 3, Line 1
The conversion of a char data type to a datetime data type resulted in an out-of-range datetime value.
The statement has been terminated.

The strange thing is that this works on a small dataset but crashes and burns on larger ones. I have tried different CAST and CONVERT options to transform the data into the correct format. I have tried a stored procedure, DTS transformations and execute SQL task in my DTS package without success.

Any help will be appreciated.
Len