I have a database running on a Raq 4 with MySQL 3.23.37 and phpMyAdmin.

The database is fine for a while (weeks) with the user adding records then after a time the db seems to 'forget' the next autoindex number, thus giving 'duplicate value in field' results when trying to add a new record.

Anyone got any bright ideas? It's driving me potty

It sounds like a problem with the script your using more than MySQL itself.
I've a number of databases that use an auto-increment and have never had any problems of this nature

blacknight, I thought that as well - but the problem even occurs if I use phpMyAdmin to insert row :-(

You've lost me then :-(
Sorry!