After slowing down to about 5mph and taking off again with the engine at normal operating temperature, the transmission (I'm assuming) makes a rather hard and audible bump. The gear shifts become rather abrupt up until overdrive when the car is loaded. At full throttle, the tranny hesitates to upshift and when it does, it does so rather harshly. Also, around right turns, the transmission does not engage right away. I have been told that this is a low fluid level issue. The fluid is full and sparkling clean. The car has almost 155,000 miles on it and no record of transmission failure or repairs. What could possibly be the issue?