From what you've said here, my suspicions fall on your kettle or developer temperature.
Originally Posted by mitomac
Platinum is actually less active than palladium (on many papers it is significantly less active) so there's no reason why it should fog more easily than palladium. However, the level of paper moisture has a significant impact on both platinum and palladium (Dmax and contrast range). A patch of paper that has more moisture than another may well fog more quickly. if you've got a fairly normal room humidity then you should be able to use paper without having to humidify it first.
When platinum is developed in a cold developer (e.g. room temperature) it can get a gritty look. You may want to heat up your developer a bit to see if this changes things. At 65C you should see greater Dmax and greater contrast (and if developer temperature is the problem then perhaps the mottling will go). Beware though that higher developer temperature will boost the palladium more than the platinum.