I am new to the board and am having tranny issues.I have a 97 Bonneville SE with 95000 miles on it. The vehicle has been maintained. I put about 1100 miles on it and decided to give it the once over. Changed tranny filter and fluid even though it looked good. Drove the car a couple thousand miles & it was fine.Now occasionally it will not shift to third. The tranny acts like it is in neutral, but if you lift off of the throttle and back on it it will go to third.This does not happen all the time.Any suggestions would be greatly appreciated.

Sticky 2-3 shift solonoid or low line pressure. Possible burned 3rd clutch, but for that you should see friction material on your dipstick after driving.

Best bet is to have a reputable transmission shop diagnose it.