Porkface is right, be wary of the full fluid change. If yours is pretty and red, and doesn't smell funny, a couple of partial changes can't hurt anything. However, since it was wierd and then it wasn't, I'd suspect you have a detent cable hanging up, and then letting go. As you are looking at your throttle, you'll see a round pulley with a cable up top, and a cable at the bottom. The cable at the bottom is the detent cable. If it looks stretched, and looks like it could loop onto that bolt you see below it to the left, that is most likely your problem. Try tugging on it and see if it moves freely or if it sticks. Often times you can lube them and make them live.

Hope this helps,
Chris