What is wrong with my dad's Peugeot 306 CD player?

My dad recently got a Peugeot 306 Estate HDi Meridian and the CD player works, it reads fine, as soon as the engine is started - boom! The CD player "freezes" and you can hear the laser trying to read it (faint click click) the numbers do not move and after about 10 seconds of not being able to read it, it spits out the CD, why? The CD player worked for the previous owner BUT the person my dad bought the car from did say that some rubber fittings were needed to stabilise the CD player? Is this correct or is the thing knackered?