All the way down at the bottom of my grid/device manager/project profile are a couple of crossproject parameters. If the switching time is too short, it can cause more swapping behavio(u)r like you mention. Mine is set to 180 minutes (3 hours) on a fairly fast computer, so that has been enough time to run a large chunk of a workunit before it switches again. I changed it a year or two ago when 60 minutes (1 hour) was a bit too short for some things, especially if the projects were not left in memory and a checkpoint was not reached.

A very small resource allocation for one BOINC project (WCG overall in our case) in comparison to other project(s) can cause this also, but usually only for the projects given the smallest piece of the pie.