As for the servo I'd guess the new servo is trying to draw too much voltage from your receiver, and if it hasn't already fried it its causing electrical interference that makes the car glitch. Test by putting the old servo back in the car. If all is well then your electronics are still OK, and you'll need to try a lower draw servo or try adding a glitch-buster capacitor to your receiver.

The battery question is simple, your cells are behaving normally. You have 6 cells, rated at 1.2V each for a total of 7.2V rating. When your pack is fully charged, the cells can read up to 1.5V each which is normal, and gives you 9V. We had several 6-cell NiMH packs that would read 10V when fresh and hot off the charger. This is referred to as the 'peak' and usually only lasts a lap or two before your cells settle to their normal voltage.