One oibservation that I would make based on your message, ZAP and DELETE ALL are very different processes. ZAP takes a whole lot less of resources than a DELETE ALL followed by a pack does.

The ZAP command:

1. Truncate the DBF file after the header record
2. Recreate the indexes

Delete all followed by a pack:

1. Mark every record in the table as deleted
2. Copy all non-deleted records to a temp table
3. Erase the original table
4. Rename the temp table to the orignal table's name
5. Recreate the indexes

So you can see that your change from using ZAP to using DELETE ALL may, in fact, be at the root of the problem you are experiencing because it causes an serious increase in network traffic and disk access. You might want to go back to using ZAP for this purpose.