Issues Shifting while going down hill?

Hey guys, I have about 45k on my 2006 X. Recently, I have noticed that if I am accellerating over the peak of a hill around 55-70 mph, my x won't shift into 4th to coast down. When let my foot off the gas. It stays in third I have to accellerate to get it to shift. This doesn't happen often, I drive about 1k miles a week and this happens mabey once every 500 mi. Any Ideas?

Have you checked the level of the transmission fluid? Has it ever been changed?