98 tranny shifting problem

interesting problem here - at only goes in reverse ~1/3; no slipping etc, either it engages or it doesn't. when it doesn't all you have to do it shift it back in park and try again. when it engages its perfect. also, check engine light comes on when shifting into OD (again only ~1/3 of the shifts) indicating general shifting problem. possible solenoid problem? pressure? any help greatly appreciated. so far, no one able to diagnosis.