sportcoupe unstable + sliding on garage ramp

>> Does anyone know if it normal for the Mercedes w203 sportcoupe to be >>unstable (on the rear) on a bumpy road? I got this problem and other
>>problem. When i drive on snow the car even in mode W (automatic gear) >>it >>keeps sliding...
>Check your shock absorbers for the bumpy problem. Tires for the snow >problem...
I've checked the suspension and it's everything ok. About the sliding, it doesn't only happen in snow.. if i try to go through my garage ramp it keeps sliding.. it only goes up if with speed..
Any ideas?
Good Luck, Marty

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