outlook 2007 slows down when ost reaches 20 GB

Running Outlook 2007, Exchange 2010 SP 2.

Outlook is running in cached mode. When the OST file reaches 20GB, outlook slows down and then becomes non responsive. If I delete the OST and let Outlook rebuild it, Outlook runs fine until the OST reaches 20GB.

I have been reading and researching and cant seem to find a fix for this. Does any know of anything?

thanks!