1998 Dodge Caravan loss of drive gears

Transmission problem
1998 Dodge Caravan 6 cyl Two Wheel Drive Automatic 249k miles

Hi, after reading through a lot of very informative content on your site I find myself having to submit this question as I cannot find anything similar here. My problem is with the transmission I believe, when I select the drive D gear it often takes 10 to seconds before it selects, then upon pulling away it jumps back out, if I rev the engine a little after several more seconds it selects again and drives. This is more imminant in the mornings, it also does the same in reverse gear ! Once it is driving it seems fine, if I have to stop sharpley though, again it loses the gear and takes several seconds to select again, this also happens when I corner or am travelling along a winding road, it seems quite bizarre ! And becomes very frustrating, I can almost anticipate when it will lose its drive gears now, its got the point now as I dont like to drive it anymore as once or twice it has lost it's gear turning or crossing intersections, to the frustration of drivers behind me also. I have been advised that the transmission needs replacing, but ive a feeling it is not the transmission at fault, perhaps a sensor ! The transmission has been well cared for, always used the correct fluid, it makes no noises at all, and when driving shifts smoothly. Hoping you can help with this. Many thanks. Daz