Limited adition is on many things that are Not. It's used to sell more. You will have to exchange  it for a new one or check the wheels for cracked gears.
This is a very common problem with GP30's. Remove the cover on the bottom of trucks and take out each wheel assembly. Look for crack in plastic gear. If you can hold the gear with your fingers and turn the wheel on the side the gear is cracked. Contact Walthers and they will send you new gears or wheels under warrenty. You can also use Athearn gears or wheels from the SD40-2.

Bob

Click and run slow should be one of the wipers on the drivers is bent. Copper strip is rubbing on spokes in the wheel like a baseball card on a bike. In forward it will rub in reverse not. Check them out.

Bob