Hi there Terry, thanks for your kind reply sir. I paint with Talens Rembrandt oils, this is Talens' finest range. I have Talens Van Gogh tubes too, this is Talens' study paint, in general they work well too. I'm sure that room temperature is important. I experience difficulties when I'm painting in a cold room. In the warm living room I get the best results. You can also store your paint in a warm environment, maybe this helps ?
You could be right on the grainy pigments, I had this problem in the past too, at that time I had Winton oils.
I'm looking forward to other members' reactions...