The only shifting problems I noticed were when I got back in the car to make a quick trip to the store. I put it in reverse, and it didn't engage at first. Again when I shifted to drive, it took a second for it to slip into gear - that was because of the loss of fluid, and subsequent loss of pressure in the transmission.

Is there a filter that needs changed? Would a flush clear any possible blockages? I'm sure that the fluid has never been changed, so it is due for new lubricant.