What is happening is simple to answer. The house is expanding and contracting with the changes in temperature. It is normal and nothing you can do to stop it. What to do with the cracks is another problem. I don't think you will ever cover them all. Paint will seal over them for a short time, but eventually the contraction/expansion will reopen the cracks. I have had some success, not 100%, patching cracks using pieces of nylon stockings under the patching plaster. Use the nylon just like you would paper or mesh tape over a joint. Those little hairline cracks are going to be with you forever.