Without the vehicle mileage, I’m guessing here. Check the power brake booster for a vacuum booster leak. I had to replace mine at 203,000 miles. Regarding the transmission not shifting into overdrive, does it shift into overdrive when it is cold, say the first 10 minutes after sitting overnight, and then drop out of overdrive? I’ve got 308,000 miles on a supercharged ’95, and the engine has never been opened. I wish I could buy another new one now.