QGIS-Project with PostGIS layers freezes irregularly for minutes

I'm using about 30 more or less complex PostGIS layer in my QGIS-Project. Some of them are PlanetOSM-Layer generated with osm2pgsql. The Project loads fast (ca 5 sec). The canvas update by moving or zooming is also fast (2sec per update). My computer have got enough resources. Overall, everything seems okay, but sometimes after doing or saving changes on a layer, switching between edit mode or by adding or removing a layer, QGIS freezes for a moment (up to 2 minutes). While hanging, one of my CPU cores is at 100%. It seems, that it does not matter, how big the layer is. Sometimes also adding a empty postgis-layer causes that freezing.

After restarting the Project, everything works fine... for the first edits.

What could be the problem?

System configuration: Xubuntu 14.04, QGIS 2.6, PostgreSQL 9.3.5, PostGIS 2.1.2 Intel i5-4570 CPU @ 3.20GHz, 8GB RAM