Harry,

You might be filling the transaction logs.

>Yes, I thought of that, but I checked permissions and I appear to have them all. I don't know if this will give anyone a clue, but I logged in to the database using "Enterprise Manager" and - initially - failed to delete even from there. Until I did it in small chunks. It would allow me to delete about 5000 rows without protest. Above that I got "unknown error".