Now sometimes it's shifts on its own and sometimes you have got to manually shift it what do you think.

 

Have you checked the transmission fluid to ensure its full of fluid and doesn't smell burnt?

 

1998 is the electronically controlled variant and there's a good chance there is some wiring issue inside the transmission or possibly to it. The transmission acts the same way when the plug is disconnected: Doesn't shift on its own but works fine with manual shifting.

 

Any codes?