G.R.
When you put her down for her nap, go through the normal soothing routine, then put her in the crib and leave her there to cry it out for up to 1 hour. She won't like it, but will soon learn that no amount of screaming will get her out of nap time. It sounds like she is testing boundaries.
It is very difficult to listen to them cry--on more than one occasion my husband found me in tears sitting outside the nursery.
Also, read Weissbluth's Healthy Sleep Habits, Happy Child. Making sure that she gets the sleep she needs is good parenting.