You might have a compatibility problem between the oil in the paint and the turpenoid. Mineral spirits comes in two varieties, regular and odorless. Try the odorless.
It could also be inadequate dispersal of the pigment and oil, which you can cure by vigorous aggitation.
Finally, smear some of the oil paint, straight from the tube, on a flat piece of glass or glazed ceramic. Feather the smear to a disappearing edge. Look for graininess there. If there is none, then it's a solvent problem. If there is, then the oil paint may be too old. (Although they should last for a very long time if kept properly.)