If it's constantly slipping and you hear that terrible grinding noise, especially when turning in grass or carpet, etc, it's the slipper clutch with the nylon gears. It is a safety feature to prevent frying the circuit board. The clutch mod that Wacky is referring to involves taking apart the gears, cleaning off all the grease, and super gluing the clutch piece to the gear to stop it from working. You run the risk of overheating the board, but no more than running the tank with metal gears. Either get used to the sound, glue the clutch as in Wacky's link, or replace with metal gears. I have two Tigers, one with plastic and one with metal gears. The metal gears are noisier than plastic. Depends what you want. The cheap fix, and if I were you, glue the clutch first and try that and save the $30+ for metal gears unless you plan on upgrading anyway. Just be careful and watch for sticks/rocks getting caught in your tracks. The clutch allows the gears to continue turning thus preventing the overload. The glued clutch/metal gears will keep trying to turn when they can't and that's when you have problems.

Good luck with it!