It's not unknown for people with neurological speech impediments to be able to talk clearly sometimes and not at other times. Stuttering is a big example--while it's usually neurological in origin, it tends to get worse whenever there's stress involved. I highly doubt your daughter is doing this deliberately; rather, I think it means that there's a lot of untapped potential for her and her speech therapist to work with. If there are some conditions--like when she is talking to her imaginary friend (essentially, to herself) that trigger clear speech, maybe she can learn to trigger those conditions at other times too.