I have an automatic 92 Firebird base 3.1L , and recently while leaving a business the car starting shifting loudly, and after stopping at a stop light I went to pick up some speed and the gears slammed causing the car to lunge forward, and then stop harshly. Luckily I idled safely into a parking lot. I can shift between each gear no problem but once I try to pick up speed (and it shifts into a higher gear) it does this. What could be the problem and possible solutions?