Alphasud,
What kind of sanding are you doing for hours at a time? Are you sculpting rough wood or sanding flat sheets? And, what grits of paper are you using? Also, how long have you used the RO 150E? Could it have been used to the point that the excess vibration is caused by worn bearings? Also, how many years have you been in the trade and does this happen when you use other hand held power tools?
I agree with the other gentleman that you may just be holding on too tight and pressing too hard as many do. Take their advice and lighten up, try a glove or both.
Do your hands get numb after any other activities, for example, while driving a vehicle?