It may be interference from the ignition coil/wires/spark plugs then. Please confirm first that you are using active low drives to the keypad, in other words one row or column, whichever you are driving is low, and the others are high. If you are using active high the noise immunity will be far less.
Can you disconnect the keypad completely so all it's wiring is eliminated and see what happens. It will help to confirm whether the inteference is travelling along the supply wiring or is being picked up by the keypad wiring. If it stops the problem you will have to put filtering on the keypad connections so please tell me whether you are driving the columns or the rows on the keypad.
Brian.