On a manual car it would probably be a worn out clutch. On an auto trans it could be a low oil level or pump problem, a torque converter fault or most commonly a slipping brake band.
does the speedometer effect the transmission
check your transmission fluid, if it is low it will slip
overheating of the fluid. This causes the transmission to slip. Slipping causes damage as friction material wears out quickly and even more overheating. This eventually causes failure.
check the fluid levels. that's the only thing I can think of.
Low transmission fluid will cause this and also weak bands inside the transmission can cause this.
You might need transmission fluid !?!?
a blown transmission
Your transmission might slip on your ML320 because the transmission fluid is low or because the transmission is damaged. The most common cause of transmission slip is wearing of the transmission.
The clutch probably just needs adjusting.
Try to replace the oil filter if the transmission is not engaging when it's cold.
The bands may be weak, it may be low on fluid.
bad linkage in shifting cable