I would be suspect of a couple things regaurding this complaint. First, the transmission valve body may be contaminated or failing due to use over time or irregular fluid and filter maintenance. A severly clogged trans filter could cause irregular shift conditions. Secondly you may have a frozen clutch pack in the trans which is preventing engagement of other gears. From your info I would probably lean towards a clutch pack. I would definatley recommend taking this to a reputable shop as trans work can get expensive if not diagnosed correctly the first time.