ML 500 Trans Slipping
 

I have a 2002 ML500 and after a few hours of driving, or 50 miles plus, the trans starts to slip a little between 3rd and 4th, but not 100% of the time, under hard acceleration it seems to be fine, under light and normal driving seems do it more often, all other shifts are fine. Is this the start of a major problem ? The control unit was changed at 40,000 miles, it now has 80,000 miles. Any advise?