So I have a 1995 olds 88 royal. It has 91k on the car and has been a great car for a daily. I purchased the car 6 months ago. Recently, the overdrive has been dropping after about 10 miles of driving and will not go back into o/d until I shut the car off and let it sit for at least 30 min. I'm changing the fluid and trans filter next weekend and I'll go from there. It has the 4l60e trans and is not the supercharged model. Any help is greatly appreciated on what could most likely be the issue, thanks