2001 CLK320 Transmission Slipping
 

My 2001 CLK320 Transmission is Slipping. Normal driving is ok, but going up a hill or grade it will go completely out of gear if you push too hard on the gas. Then if you let off the gas or pull over and stop it will go back in gear. I found out the transmission fluid is full, after buying a dipstick online. Any ideas as to how to fix the problem, short of getting a new trans?