SSIS package suddenly takes 90 seconds longer to run

I have started to experience a problem with SSIS. I have a number of packages that read from an excel file into a SQL staging table. For about six months, they took three or four seconds to run and were always successful (as long as nobody had the excel file open).

Three weeks ago they all started taking ninety two or ninety three seconds to run. I had not modified or redeployed any packages around the time the problem occured. The packages still complete successfully.

I have copied the excel file to the SQL Server to rule out network issues and there is no difference.
The package still only takes three or four seconds to run on my (BIDS) development server.
I call the packages using a stored procedure and when I run this in a query window I see the following (note the time difference between started and progress lines):

Microsoft (R) SQL Server Execute Package Utility
Version 9.00.3042.00 for 32-bit
Copyright (C) Microsoft Corp 1984-2005. All rights reserved.
NULL
Started: 14:11:32
Progress: 2009-02-02 14:13:03.31
Source: Import Sales
Validating: 0% complete

etc., etc.

I have seen this problem reported elsewhere but nobody appears to have supplied an answer.

My SQL Server is 64bit Enterprise and I develop in BIDS on a 32bit server.