Import with Update Matching

[fmsavey]

I routinely run an import with update on matching fields and add new records a couple of times a day. Only one field is specified as a match. Typically I import about 3000 records from an ODBC source. Problem is... after a couple of imports, it begins to slow down from several hundred records per second to one record per second. At that point it is taking 50 minutes to import all the records. This is just unacceptable. I don't understand why this is happening. When I recover the file the performance picks up but then after one or two updates it back to a crawl. HELP!