I get a foaming or frothing effect when using a roller. What causes this?

You're using the wrong type of roller. For instance, if you use a sponge roller to apply water-based paints, air is injected into the wet paint film and the air bubbles burst, forming craters on the surface which dry unevenly. As a rule of thumb, use short pile rollers for flat surfaces, medium pile rollers for medium textures and long pile rollers for textured surfaces. After rolling, some paints, particularly solvent-based gloss, may need to be 'layed-off' with a brush to eliminate foaming. To repair a foamy surface, thoroughly clean it down to remove all dirt, grease and surface contaminants. Rub down the surfaces with wet and dry abrasion and water or a suitable solvent. Finally, rinse down and allow to dry thoroughly before repainting. If this is impractical, line the walls before repainting.