Possible shifting issues

[IIv SHaDoW vII]

I noticed sometimes while accelerating from a stop as my car gets up to speed and shifts to third gear, about 2-5 seconds later will downshift to a lower gear again. This is while I'm on flat surface, and I also make sure to keep my acceleration at a steady pace while driving.. The tranny fluid is fine (hot mark while hot) and the transmission has never slipped. Its just when it gets to that third gear and accelerating at low rpms it sometimes shifts down again.. sometimes it will shift back down after about 2 seconds.. other times it will shift down after about 4 - 5 seconds. Sometimes it wont shift down again at all.. but thats usually when I am not holding the accelerator. I am not trying to say my transmission wont shift up or down.. It downshifts fine when coming to a stop and it upshifts fine when driving through the gears. The only thing is when it hits third gear it downshifts again and then shifts back to third when I am done accelerating. Is this normal? Am I being paranoid?