I have try to do some experiment and i have find than if i measure the output voltage from lm35 with a multimeter i get the correct one. So the problem is not in the probe(thank's Mark for the tips with capacitor) but in adc reading of my 16f676 :-(
The strange thing is if i try to take some measure with a trimmer tied from vcc and gnd i will get always the correct and stable measure but when i try do measure the output voltage from lm35 i read different voltage variations +/- 2v and the measure is very unstable. I don't know more what i can try for have a decent measure from lm35.