There's a lot of electrical rubbish flying about in a car - you need to filter the incoming supply particularly well, and fitting the project in a screen box is desirable. Also screen the leads the the sensor, and keep them as short as possible.
First off I would use a voltage regulator to feed the circuit, dropping the supply to 9V or 6V through a regulator would remove a lot of PSU noise. Then feed that from the 12V car supply through a low-pass filter, with a series choke and capacitors down to ground on both sides.
On thing you might try first, running it off a totally separate supply, use an external dry battery to power it, and see if it helps.
ok, I'll shield everything this time, and use a voltage regulator.
i may also try a high quality audio type car noise filter, as I'm not familiar with filtering.
ta once again.