Hi Richard,
I think this is happening because of the load on the memory to process the overheads as the day passes by.I think as the day passes the amount of data increases and as because somehow the overhead for the memory process increases and that is why it taking more time.
Some suggessions to look at:
open only those file which is essential,otherwise open and do the operations and close it.
don't try to pack or reindex often during the running of the application.If the database is large the overhead time for these goes up and it might eatup your processing time.
Try to monitor the memory absorbtion in the course of the running application
Raj